[PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

[PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

[Ahmed S. Darwish]

Hi,

Changelog-v2
============

  - Standardize on seqcount_LOCKNAME_t as the canonical reference for
    sequence counters with associated locks, instead of v1
    seqcount_LOCKTYPE_t.

  - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t
    property accessors.

  - Touch-up the lock-unlock rationale for more clarity. Enforce writer
    non-preemitiblity using "__seq_enforce_writer_non_preemptibility()".

Cover letter (v1)
=================

https://lkml.kernel.org/r/20200828010710.5407-1-a.darwish@linutronix.de

Preemption must be disabled before entering a sequence counter write
side critical section.  Otherwise the read side section can preempt the
write side section and spin for the entire scheduler tick.  If that
reader belongs to a real-time scheduling class, it can spin forever and
the kernel will livelock.

Disabling preemption cannot be done for PREEMPT_RT though: it can lead
to higher latencies, and the write side sections will not be able to
acquire locks which become sleeping locks (e.g. spinlock_t).

To remain preemptible, while avoiding a possible livelock caused by the
reader preempting the writer, use a different technique: let the reader
detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
case, acquire then release the associated LOCKNAME writer serialization
lock. This will allow any possibly-preempted writer to make progress
until the end of its writer serialization lock critical section.

Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
an associated (PREEMPT_RT) sleeping lock, and for seqlock_t.

8<--------------

Ahmed S. Darwish (5):
  seqlock: seqcount_LOCKNAME_t: Standardize naming convention
  seqlock: Use unique prefix for seqcount_t property accessors
  seqlock: seqcount_t: Implement all read APIs as statement expressions
  seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support
  seqlock: PREEMPT_RT: Do not starve seqlock_t writers

 include/linux/seqlock.h | 281 ++++++++++++++++++++++++----------------
 1 file changed, 167 insertions(+), 114 deletions(-)

base-commit: f75aef392f869018f78cfedf3c320a6b3fcfda6b
--
2.28.0

[PATCH v2 1/5] seqlock: seqcount_LOCKNAME_t: Standardize naming convention

[Ahmed S. Darwish]

At seqlock.h, sequence counters with associated locks are either called
seqcount_LOCKNAME_t, seqcount_LOCKTYPE_t, or seqcount_locktype_t.

Standardize on seqcount_LOCKNAME_t for all instances in comments,
kernel-doc, and SEQCOUNT_LOCKNAME() generative macro paramters.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
---
 include/linux/seqlock.h | 83 +++++++++++++++++++++--------------------
 1 file changed, 42 insertions(+), 41 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 962d9768945f..4f219df659b1 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -53,7 +53,7 @@
  *
  * If the write serialization mechanism is one of the common kernel
  * locking primitives, use a sequence counter with associated lock
- * (seqcount_LOCKTYPE_t) instead.
+ * (seqcount_LOCKNAME_t) instead.
  *
  * If it's desired to automatically handle the sequence counter writer
  * serialization and non-preemptibility requirements, use a sequential
@@ -117,7 +117,7 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
 #define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
 
 /*
- * Sequence counters with associated locks (seqcount_LOCKTYPE_t)
+ * Sequence counters with associated locks (seqcount_LOCKNAME_t)
  *
  * A sequence counter which associates the lock used for writer
  * serialization at initialization time. This enables lockdep to validate
@@ -138,30 +138,32 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
 #endif
 
 /**
- * typedef seqcount_LOCKNAME_t - sequence counter with LOCKTYPR associated
+ * typedef seqcount_LOCKNAME_t - sequence counter with LOCKNAME associated
  * @seqcount:	The real sequence counter
- * @lock:	Pointer to the associated spinlock
+ * @lock:	Pointer to the associated lock
  *
- * A plain sequence counter with external writer synchronization by a
- * spinlock. The spinlock is associated to the sequence count in the
+ * A plain sequence counter with external writer synchronization by
+ * LOCKNAME @lock. The lock is associated to the sequence counter in the
  * static initializer or init function. This enables lockdep to validate
  * that the write side critical section is properly serialized.
+ *
+ * LOCKNAME:	raw_spinlock, spinlock, rwlock, mutex, or ww_mutex.
  */
 
 /**
  * seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t
  * @s:		Pointer to the seqcount_LOCKNAME_t instance
- * @lock:	Pointer to the associated LOCKTYPE
+ * @lock:	Pointer to the associated lock
  */
 
 /*
- * SEQCOUNT_LOCKTYPE() - Instantiate seqcount_LOCKNAME_t and helpers
- * @locktype:		actual typename
- * @lockname:		name
- * @preemptible:	preemptibility of above locktype
+ * SEQCOUNT_LOCKNAME() - Instantiate seqcount_LOCKNAME_t and helpers
+ * @lockname:		"LOCKNAME" part of seqcount_LOCKNAME_t
+ * @locktype:		LOCKNAME canonical C data type
+ * @preemptible:	preemptibility of above lockname
  * @lockmember:		argument for lockdep_assert_held()
  */
-#define SEQCOUNT_LOCKTYPE(locktype, lockname, preemptible, lockmember)	\
+#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember)	\
 typedef struct seqcount_##lockname {					\
 	seqcount_t		seqcount;				\
 	__SEQ_LOCK(locktype	*lock);					\
@@ -211,29 +213,28 @@ static inline void __seqcount_assert(seqcount_t *s)
 	lockdep_assert_preemption_disabled();
 }
 
-SEQCOUNT_LOCKTYPE(raw_spinlock_t,	raw_spinlock,	false,	s->lock)
-SEQCOUNT_LOCKTYPE(spinlock_t,		spinlock,	false,	s->lock)
-SEQCOUNT_LOCKTYPE(rwlock_t,		rwlock,		false,	s->lock)
-SEQCOUNT_LOCKTYPE(struct mutex,		mutex,		true,	s->lock)
-SEQCOUNT_LOCKTYPE(struct ww_mutex,	ww_mutex,	true,	&s->lock->base)
+SEQCOUNT_LOCKNAME(raw_spinlock,	raw_spinlock_t,		false,	s->lock)
+SEQCOUNT_LOCKNAME(spinlock,	spinlock_t,		false,	s->lock)
+SEQCOUNT_LOCKNAME(rwlock,	rwlock_t,		false,	s->lock)
+SEQCOUNT_LOCKNAME(mutex,	struct mutex,		true,	s->lock)
+SEQCOUNT_LOCKNAME(ww_mutex,	struct ww_mutex,	true,	&s->lock->base)
 
 /**
  * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
  * @name:	Name of the seqcount_LOCKNAME_t instance
- * @lock:	Pointer to the associated LOCKTYPE
+ * @lock:	Pointer to the associated LOCKNAME
  */
 
-#define SEQCOUNT_LOCKTYPE_ZERO(seq_name, assoc_lock) {			\
+#define SEQCOUNT_LOCKNAME_ZERO(seq_name, assoc_lock) {			\
 	.seqcount		= SEQCNT_ZERO(seq_name.seqcount),	\
 	__SEQ_LOCK(.lock	= (assoc_lock))				\
 }
 
-#define SEQCNT_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKTYPE_ZERO(name, lock)
-#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKTYPE_ZERO(name, lock)
-#define SEQCNT_RWLOCK_ZERO(name, lock)		SEQCOUNT_LOCKTYPE_ZERO(name, lock)
-#define SEQCNT_MUTEX_ZERO(name, lock)		SEQCOUNT_LOCKTYPE_ZERO(name, lock)
-#define SEQCNT_WW_MUTEX_ZERO(name, lock) 	SEQCOUNT_LOCKTYPE_ZERO(name, lock)
-
+#define SEQCNT_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name, lock)
+#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name, lock)
+#define SEQCNT_RWLOCK_ZERO(name, lock)		SEQCOUNT_LOCKNAME_ZERO(name, lock)
+#define SEQCNT_MUTEX_ZERO(name, lock)		SEQCOUNT_LOCKNAME_ZERO(name, lock)
+#define SEQCNT_WW_MUTEX_ZERO(name, lock) 	SEQCOUNT_LOCKNAME_ZERO(name, lock)
 
 #define __seqprop_case(s, lockname, prop)				\
 	seqcount_##lockname##_t: __seqcount_##lockname##_##prop((void *)(s))
@@ -252,7 +253,7 @@ SEQCOUNT_LOCKTYPE(struct ww_mutex,	ww_mutex,	true,	&s->lock->base)
 
 /**
  * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -283,7 +284,7 @@ static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
 
 /**
  * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * Return: count to be passed to read_seqcount_retry()
  */
@@ -299,7 +300,7 @@ static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
 
 /**
  * read_seqcount_begin() - begin a seqcount_t read critical section
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * Return: count to be passed to read_seqcount_retry()
  */
@@ -314,7 +315,7 @@ static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
 
 /**
  * raw_read_seqcount() - read the raw seqcount_t counter value
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * raw_read_seqcount opens a read critical section of the given
  * seqcount_t, without any lockdep checking, and without checking or
@@ -337,7 +338,7 @@ static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
 /**
  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
  *                        lockdep and w/o counter stabilization
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * raw_seqcount_begin opens a read critical section of the given
  * seqcount_t. Unlike read_seqcount_begin(), this function will not wait
@@ -365,7 +366,7 @@ static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
 
 /**
  * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  * @start: count, from read_seqcount_begin()
  *
  * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
@@ -389,7 +390,7 @@ static inline int __read_seqcount_t_retry(const seqcount_t *s, unsigned start)
 
 /**
  * read_seqcount_retry() - end a seqcount_t read critical section
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  * @start: count, from read_seqcount_begin()
  *
  * read_seqcount_retry closes the read critical section of given
@@ -409,7 +410,7 @@ static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
 
 /**
  * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  */
 #define raw_write_seqcount_begin(s)					\
 do {									\
@@ -428,7 +429,7 @@ static inline void raw_write_seqcount_t_begin(seqcount_t *s)
 
 /**
  * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  */
 #define raw_write_seqcount_end(s)					\
 do {									\
@@ -448,7 +449,7 @@ static inline void raw_write_seqcount_t_end(seqcount_t *s)
 /**
  * write_seqcount_begin_nested() - start a seqcount_t write section with
  *                                 custom lockdep nesting level
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  * @subclass: lockdep nesting level
  *
  * See Documentation/locking/lockdep-design.rst
@@ -471,7 +472,7 @@ static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
 
 /**
  * write_seqcount_begin() - start a seqcount_t write side critical section
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * write_seqcount_begin opens a write side critical section of the given
  * seqcount_t.
@@ -497,7 +498,7 @@ static inline void write_seqcount_t_begin(seqcount_t *s)
 
 /**
  * write_seqcount_end() - end a seqcount_t write side critical section
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * The write section must've been opened with write_seqcount_begin().
  */
@@ -517,7 +518,7 @@ static inline void write_seqcount_t_end(seqcount_t *s)
 
 /**
  * raw_write_seqcount_barrier() - do a seqcount_t write barrier
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * This can be used to provide an ordering guarantee instead of the usual
  * consistency guarantee. It is one wmb cheaper, because it can collapse
@@ -571,7 +572,7 @@ static inline void raw_write_seqcount_t_barrier(seqcount_t *s)
 /**
  * write_seqcount_invalidate() - invalidate in-progress seqcount_t read
  *                               side operations
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * After write_seqcount_invalidate, no seqcount_t read side operations
  * will complete successfully and see data older than this.
@@ -589,7 +590,7 @@ static inline void write_seqcount_t_invalidate(seqcount_t *s)
 
 /**
  * raw_read_seqcount_latch() - pick even/odd seqcount_t latch data copy
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * Use seqcount_t latching to switch between two storage places protected
  * by a sequence counter. Doing so allows having interruptible, preemptible,
@@ -614,7 +615,7 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s)
 
 /**
  * raw_write_seqcount_latch() - redirect readers to even/odd copy
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * The latch technique is a multiversion concurrency control method that allows
  * queries during non-atomic modifications. If you can guarantee queries never
-- 
2.28.0

[PATCH v2 2/5] seqlock: Use unique prefix for seqcount_t property accessors

[Ahmed S. Darwish]

At seqlock.h, the following set of functions:

    - __seqcount_ptr()
    - __seqcount_preemptible()
    - __seqcount_assert()

act as plain seqcount_t "property" accessors. Meanwhile, the following
group:

    - __seqcount_ptr()
    - __seqcount_lock_preemptible()
    - __seqcount_assert_lock_held()

act as the equivalent set, but in the generic form, taking either
seqcount_t or any of the seqcount_LOCKNAME_t variants.

This is quite confusing, especially the first member where it is called
exactly the same in both groups.

Differentiate the first group by using "__seqprop" as prefix, and also
use that same prefix for all of seqcount_LOCKNAME_t property accessors.

While at it, constify the property accessors first parameter when
appropriate.

References: 55f3560df975 ("seqlock: Extend seqcount API with associated locks")
Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
---
 include/linux/seqlock.h | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 4f219df659b1..96198da7debc 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -157,7 +157,9 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
  */
 
 /*
- * SEQCOUNT_LOCKNAME() - Instantiate seqcount_LOCKNAME_t and helpers
+ * SEQCOUNT_LOCKNAME()           - Instantiate seqcount_LOCKNAME_t and helpers
+ * seqprop_seqcount_LOCKNAME_*() - Property accessors for seqcount_LOCKNAME_t
+ *
  * @lockname:		"LOCKNAME" part of seqcount_LOCKNAME_t
  * @locktype:		LOCKNAME canonical C data type
  * @preemptible:	preemptibility of above lockname
@@ -177,19 +179,19 @@ seqcount_##lockname##_init(seqcount_##lockname##_t *s, locktype *lock)	\
 }									\
 									\
 static __always_inline seqcount_t *					\
-__seqcount_##lockname##_ptr(seqcount_##lockname##_t *s)			\
+__seqprop_seqcount_##lockname##_ptr(seqcount_##lockname##_t *s)		\
 {									\
 	return &s->seqcount;						\
 }									\
 									\
 static __always_inline bool						\
-__seqcount_##lockname##_preemptible(seqcount_##lockname##_t *s)		\
+__seqprop_seqcount_##lockname##_preemptible(const seqcount_##lockname##_t *s)\
 {									\
 	return preemptible;						\
 }									\
 									\
 static __always_inline void						\
-__seqcount_##lockname##_assert(seqcount_##lockname##_t *s)		\
+__seqprop_seqcount_##lockname##_assert(const seqcount_##lockname##_t *s)\
 {									\
 	__SEQ_LOCK(lockdep_assert_held(lockmember));			\
 }
@@ -198,17 +200,17 @@ __seqcount_##lockname##_assert(seqcount_##lockname##_t *s)		\
  * __seqprop() for seqcount_t
  */
 
-static inline seqcount_t *__seqcount_ptr(seqcount_t *s)
+static inline seqcount_t *__seqprop_seqcount_ptr(seqcount_t *s)
 {
 	return s;
 }
 
-static inline bool __seqcount_preemptible(seqcount_t *s)
+static inline bool __seqprop_seqcount_preemptible(const seqcount_t *s)
 {
 	return false;
 }
 
-static inline void __seqcount_assert(seqcount_t *s)
+static inline void __seqprop_seqcount_assert(const seqcount_t *s)
 {
 	lockdep_assert_preemption_disabled();
 }
@@ -237,10 +239,10 @@ SEQCOUNT_LOCKNAME(ww_mutex,	struct ww_mutex,	true,	&s->lock->base)
 #define SEQCNT_WW_MUTEX_ZERO(name, lock) 	SEQCOUNT_LOCKNAME_ZERO(name, lock)
 
 #define __seqprop_case(s, lockname, prop)				\
-	seqcount_##lockname##_t: __seqcount_##lockname##_##prop((void *)(s))
+	seqcount_##lockname##_t: __seqprop_seqcount_##lockname##_##prop((void *)(s))
 
 #define __seqprop(s, prop) _Generic(*(s),				\
-	seqcount_t:		__seqcount_##prop((void *)(s)),		\
+	seqcount_t:		__seqprop_seqcount_##prop((void *)(s)),	\
 	__seqprop_case((s),	raw_spinlock,	prop),			\
 	__seqprop_case((s),	spinlock,	prop),			\
 	__seqprop_case((s),	rwlock,		prop),			\
-- 
2.28.0

[PATCH v2 3/5] seqlock: seqcount_t: Implement all read APIs as statement expressions

[Ahmed S. Darwish]

The sequence counters read APIs are implemented as CPP macros, so they
can take either seqcount_t or any of the seqcount_LOCKNAME_t variants.
Such macros then get *directly* transformed to internal C functions that
only take plain seqcount_t.

Further commits need access to seqcount_LOCKNAME_t inside of the actual
read APIs code. Thus transform all of the seqcount read APIs to pure GCC
statement expressions instead.

This will not break type-safety: all of the transformed APIs resolve to
a _Generic() selection that does not have a "default" case.

This will also not affect the transformed APIs readability: previously
added kernel-doc above all of seqlock.h functions makes the expectations
quite clear for call-site developers.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
---
 include/linux/seqlock.h | 94 ++++++++++++++++++++---------------------
 1 file changed, 45 insertions(+), 49 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 96198da7debc..ed1c6c0ff8bb 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -184,6 +184,12 @@ __seqprop_seqcount_##lockname##_ptr(seqcount_##lockname##_t *s)		\
 	return &s->seqcount;						\
 }									\
 									\
+static __always_inline unsigned						\
+__seqprop_seqcount_##lockname##_sequence(const seqcount_##lockname##_t *s)\
+{									\
+	return READ_ONCE(s->seqcount.sequence);				\
+}									\
+									\
 static __always_inline bool						\
 __seqprop_seqcount_##lockname##_preemptible(const seqcount_##lockname##_t *s)\
 {									\
@@ -205,6 +211,11 @@ static inline seqcount_t *__seqprop_seqcount_ptr(seqcount_t *s)
 	return s;
 }
 
+static inline unsigned __seqprop_seqcount_sequence(const seqcount_t *s)
+{
+	return READ_ONCE(s->sequence);
+}
+
 static inline bool __seqprop_seqcount_preemptible(const seqcount_t *s)
 {
 	return false;
@@ -250,6 +261,7 @@ SEQCOUNT_LOCKNAME(ww_mutex,	struct ww_mutex,	true,	&s->lock->base)
 	__seqprop_case((s),	ww_mutex,	prop))
 
 #define __seqcount_ptr(s)		__seqprop(s, ptr)
+#define __seqcount_sequence(s)		__seqprop(s, sequence)
 #define __seqcount_lock_preemptible(s)	__seqprop(s, preemptible)
 #define __seqcount_assert_lock_held(s)	__seqprop(s, assert)
 
@@ -268,21 +280,15 @@ SEQCOUNT_LOCKNAME(ww_mutex,	struct ww_mutex,	true,	&s->lock->base)
  * Return: count to be passed to read_seqcount_retry()
  */
 #define __read_seqcount_begin(s)					\
-	__read_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
-{
-	unsigned ret;
-
-repeat:
-	ret = READ_ONCE(s->sequence);
-	if (unlikely(ret & 1)) {
-		cpu_relax();
-		goto repeat;
-	}
-	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
-	return ret;
-}
+({									\
+	unsigned seq;							\
+									\
+	while ((seq = __seqcount_sequence(s)) & 1)			\
+		cpu_relax();						\
+									\
+	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);			\
+	seq;								\
+})
 
 /**
  * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
@@ -291,14 +297,12 @@ static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
  * Return: count to be passed to read_seqcount_retry()
  */
 #define raw_read_seqcount_begin(s)					\
-	raw_read_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
-{
-	unsigned ret = __read_seqcount_t_begin(s);
-	smp_rmb();
-	return ret;
-}
+({									\
+	unsigned seq = __read_seqcount_begin(s);			\
+									\
+	smp_rmb();							\
+	seq;								\
+})
 
 /**
  * read_seqcount_begin() - begin a seqcount_t read critical section
@@ -307,13 +311,10 @@ static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
  * Return: count to be passed to read_seqcount_retry()
  */
 #define read_seqcount_begin(s)						\
-	read_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
-{
-	seqcount_lockdep_reader_access(s);
-	return raw_read_seqcount_t_begin(s);
-}
+({									\
+	seqcount_lockdep_reader_access(__seqcount_ptr(s));		\
+	raw_read_seqcount_begin(s);					\
+})
 
 /**
  * raw_read_seqcount() - read the raw seqcount_t counter value
@@ -327,15 +328,13 @@ static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
  * Return: count to be passed to read_seqcount_retry()
  */
 #define raw_read_seqcount(s)						\
-	raw_read_seqcount_t(__seqcount_ptr(s))
-
-static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
-{
-	unsigned ret = READ_ONCE(s->sequence);
-	smp_rmb();
-	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
-	return ret;
-}
+({									\
+	unsigned seq = __seqcount_sequence(s);				\
+									\
+	smp_rmb();							\
+	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);			\
+	seq;								\
+})
 
 /**
  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
@@ -355,16 +354,13 @@ static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
  * Return: count to be passed to read_seqcount_retry()
  */
 #define raw_seqcount_begin(s)						\
-	raw_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
-{
-	/*
-	 * If the counter is odd, let read_seqcount_retry() fail
-	 * by decrementing the counter.
-	 */
-	return raw_read_seqcount_t(s) & ~1;
-}
+({									\
+	/*								\
+	 * If the counter is odd, let read_seqcount_retry() fail	\
+	 * by decrementing the counter.					\
+	 */								\
+	raw_read_seqcount(s) & ~1;					\
+})
 
 /**
  * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
-- 
2.28.0

[PATCH v2 4/5] seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support

[Ahmed S. Darwish]

Preemption must be disabled before entering a sequence counter write
side critical section.  Otherwise the read side section can preempt the
write side section and spin for the entire scheduler tick.  If that
reader belongs to a real-time scheduling class, it can spin forever and
the kernel will livelock.

Disabling preemption cannot be done for PREEMPT_RT though: it can lead
to higher latencies, and the write side sections will not be able to
acquire locks which become sleeping locks (e.g. spinlock_t).

To remain preemptible, while avoiding a possible livelock caused by the
reader preempting the writer, use a different technique: let the reader
detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
case, acquire then release the associated LOCKNAME writer serialization
lock. This will allow any possibly-preempted writer to make progress
until the end of its writer serialization lock critical section.

Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
an associated (PREEMPT_RT) sleeping lock.

Link: https://lkml.kernel.org/r/159708609435.2571.13948681727529247231.tglx@nanos
Link: https://lkml.kernel.org/r/20200519214547.352050-1-a.darwish@linutronix.de
References: 55f3560df975 ("seqlock: Extend seqcount API with associated locks")
Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
---
 include/linux/seqlock.h | 72 +++++++++++++++++++++++++++++++++--------
 1 file changed, 58 insertions(+), 14 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index ed1c6c0ff8bb..6ac5a63fc536 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -17,6 +17,7 @@
 #include <linux/kcsan-checks.h>
 #include <linux/lockdep.h>
 #include <linux/mutex.h>
+#include <linux/ww_mutex.h>
 #include <linux/preempt.h>
 #include <linux/spinlock.h>
 
@@ -131,7 +132,23 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
  * See Documentation/locking/seqlock.rst
  */
 
-#ifdef CONFIG_LOCKDEP
+/*
+ * For PREEMPT_RT, seqcount_LOCKNAME_t write side critical sections cannot
+ * disable preemption. It can lead to higher latencies, and the write side
+ * sections will not be able to acquire locks which become sleeping locks
+ * (e.g. spinlock_t).
+ *
+ * To remain preemptible while avoiding a possible livelock caused by the
+ * reader preempting the writer, use a different technique: let the reader
+ * detect if a seqcount_LOCKNAME_t writer is in progress. If that is the
+ * case, acquire then release the associated LOCKNAME writer serialization
+ * lock. This will allow any possibly-preempted writer to make progress
+ * until the end of its writer serialization lock critical section.
+ *
+ * This lock-unlock technique must be implemented for all of PREEMPT_RT
+ * sleeping locks.  See Documentation/locking/locktypes.rst
+ */
+#if defined(CONFIG_LOCKDEP) || defined(CONFIG_PREEMPT_RT)
 #define __SEQ_LOCK(expr)	expr
 #else
 #define __SEQ_LOCK(expr)
@@ -162,10 +179,12 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
  *
  * @lockname:		"LOCKNAME" part of seqcount_LOCKNAME_t
  * @locktype:		LOCKNAME canonical C data type
- * @preemptible:	preemptibility of above lockname
+ * @preemptible:	preemptibility of above locktype
  * @lockmember:		argument for lockdep_assert_held()
+ * @lockbase:		associated lock release function (prefix only)
+ * @lock_acquire:	associated lock acquisition function (full call)
  */
-#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember)	\
+#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember, lockbase, lock_acquire) \
 typedef struct seqcount_##lockname {					\
 	seqcount_t		seqcount;				\
 	__SEQ_LOCK(locktype	*lock);					\
@@ -187,7 +206,23 @@ __seqprop_seqcount_##lockname##_ptr(seqcount_##lockname##_t *s)		\
 static __always_inline unsigned						\
 __seqprop_seqcount_##lockname##_sequence(const seqcount_##lockname##_t *s)\
 {									\
-	return READ_ONCE(s->seqcount.sequence);				\
+	unsigned seq = READ_ONCE(s->seqcount.sequence);			\
+									\
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))				\
+		return seq;						\
+									\
+	if (preemptible && unlikely(seq & 1)) {				\
+		__SEQ_LOCK(lock_acquire);				\
+		__SEQ_LOCK(lockbase##_unlock(s->lock));			\
+									\
+		/*							\
+		 * Re-read the sequence counter since the (possibly	\
+		 * preempted) writer made progress.			\
+		 */							\
+		seq = READ_ONCE(s->seqcount.sequence);			\
+	}								\
+									\
+	return seq;							\
 }									\
 									\
 static __always_inline bool						\
@@ -226,11 +261,13 @@ static inline void __seqprop_seqcount_assert(const seqcount_t *s)
 	lockdep_assert_preemption_disabled();
 }
 
-SEQCOUNT_LOCKNAME(raw_spinlock,	raw_spinlock_t,		false,	s->lock)
-SEQCOUNT_LOCKNAME(spinlock,	spinlock_t,		false,	s->lock)
-SEQCOUNT_LOCKNAME(rwlock,	rwlock_t,		false,	s->lock)
-SEQCOUNT_LOCKNAME(mutex,	struct mutex,		true,	s->lock)
-SEQCOUNT_LOCKNAME(ww_mutex,	struct ww_mutex,	true,	&s->lock->base)
+#define __SEQ_RT	IS_ENABLED(CONFIG_PREEMPT_RT)
+
+SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t,  false,    s->lock,        raw_spin, raw_spin_lock(s->lock))
+SEQCOUNT_LOCKNAME(spinlock,     spinlock_t,      __SEQ_RT, s->lock,        spin,     spin_lock(s->lock))
+SEQCOUNT_LOCKNAME(rwlock,       rwlock_t,        __SEQ_RT, s->lock,        read,     read_lock(s->lock))
+SEQCOUNT_LOCKNAME(mutex,        struct mutex,    true,     s->lock,        mutex,    mutex_lock(s->lock))
+SEQCOUNT_LOCKNAME(ww_mutex,     struct ww_mutex, true,     &s->lock->base, ww_mutex, ww_mutex_lock(s->lock, NULL))
 
 /**
  * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
@@ -406,13 +443,20 @@ static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
 	return __read_seqcount_t_retry(s, start);
 }
 
+/*
+ * Enforce non-preemptibility for all seqcount_LOCKNAME_t writers. Don't
+ * do it for PREEMPT_RT, for the reasons outlined at __SEQ_LOCK().
+ */
+#define __seq_enforce_writer_non_preemptibility(s)			\
+	(!IS_ENABLED(CONFIG_PREEMPT_RT) && __seqcount_lock_preemptible(s))
+
 /**
  * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  */
 #define raw_write_seqcount_begin(s)					\
 do {									\
-	if (__seqcount_lock_preemptible(s))				\
+	if (__seq_enforce_writer_non_preemptibility(s))			\
 		preempt_disable();					\
 									\
 	raw_write_seqcount_t_begin(__seqcount_ptr(s));			\
@@ -433,7 +477,7 @@ static inline void raw_write_seqcount_t_begin(seqcount_t *s)
 do {									\
 	raw_write_seqcount_t_end(__seqcount_ptr(s));			\
 									\
-	if (__seqcount_lock_preemptible(s))				\
+	if (__seq_enforce_writer_non_preemptibility(s))			\
 		preempt_enable();					\
 } while (0)
 
@@ -456,7 +500,7 @@ static inline void raw_write_seqcount_t_end(seqcount_t *s)
 do {									\
 	__seqcount_assert_lock_held(s);					\
 									\
-	if (__seqcount_lock_preemptible(s))				\
+	if (__seq_enforce_writer_non_preemptibility(s))			\
 		preempt_disable();					\
 									\
 	write_seqcount_t_begin_nested(__seqcount_ptr(s), subclass);	\
@@ -483,7 +527,7 @@ static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
 do {									\
 	__seqcount_assert_lock_held(s);					\
 									\
-	if (__seqcount_lock_preemptible(s))				\
+	if (__seq_enforce_writer_non_preemptibility(s))			\
 		preempt_disable();					\
 									\
 	write_seqcount_t_begin(__seqcount_ptr(s));			\
@@ -504,7 +548,7 @@ static inline void write_seqcount_t_begin(seqcount_t *s)
 do {									\
 	write_seqcount_t_end(__seqcount_ptr(s));			\
 									\
-	if (__seqcount_lock_preemptible(s))				\
+	if (__seq_enforce_writer_non_preemptibility(s))			\
 		preempt_enable();					\
 } while (0)
 
-- 
2.28.0

[PATCH v2 5/5] seqlock: PREEMPT_RT: Do not starve seqlock_t writers

[Ahmed S. Darwish]

On PREEMPT_RT, seqlock_t is transformed to a sleeping lock that do not
disable preemption. A seqlock_t reader can thus preempt its write side
section and spin for the enter scheduler tick. If that reader belongs to
a real-time scheduling class, it can spin forever and the kernel will
livelock.

To break this livelock possibility on PREEMPT_RT, implement seqlock_t in
terms of "seqcount_spinlock_t" instead of plain "seqcount_t".

Beside its pure annotational value, this will leverage the existing
seqcount_LOCKNAME_T PREEMPT_RT anti-livelock mechanisms, without adding
any extra code.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
---
 include/linux/seqlock.h | 32 +++++++++++++++++++++-----------
 1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 6ac5a63fc536..06a339355c3a 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -757,13 +757,17 @@ static inline void raw_write_seqcount_t_latch(seqcount_t *s)
  *    - Documentation/locking/seqlock.rst
  */
 typedef struct {
-	struct seqcount seqcount;
+	/*
+	 * Make sure that readers don't starve writers on PREEMPT_RT: use
+	 * seqcount_spinlock_t instead of seqcount_t. Check __SEQ_LOCK().
+	 */
+	seqcount_spinlock_t seqcount;
 	spinlock_t lock;
 } seqlock_t;
 
 #define __SEQLOCK_UNLOCKED(lockname)					\
 	{								\
-		.seqcount = SEQCNT_ZERO(lockname),			\
+		.seqcount = SEQCNT_SPINLOCK_ZERO(lockname, &(lockname).lock), \
 		.lock =	__SPIN_LOCK_UNLOCKED(lockname)			\
 	}
 
@@ -773,8 +777,8 @@ typedef struct {
  */
 #define seqlock_init(sl)						\
 	do {								\
-		seqcount_init(&(sl)->seqcount);				\
 		spin_lock_init(&(sl)->lock);				\
+		seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock);	\
 	} while (0)
 
 /**
@@ -821,6 +825,12 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
 	return read_seqcount_retry(&sl->seqcount, start);
 }
 
+/*
+ * For all seqlock_t write side functions, use write_seqcount_*t*_begin()
+ * instead of the generic write_seqcount_begin(). This way, no redundant
+ * lockdep_assert_held() checks are added.
+ */
+
 /**
  * write_seqlock() - start a seqlock_t write side critical section
  * @sl: Pointer to seqlock_t
@@ -837,7 +847,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
 static inline void write_seqlock(seqlock_t *sl)
 {
 	spin_lock(&sl->lock);
-	write_seqcount_t_begin(&sl->seqcount);
+	write_seqcount_t_begin(&sl->seqcount.seqcount);
 }
 
 /**
@@ -849,7 +859,7 @@ static inline void write_seqlock(seqlock_t *sl)
  */
 static inline void write_sequnlock(seqlock_t *sl)
 {
-	write_seqcount_t_end(&sl->seqcount);
+	write_seqcount_t_end(&sl->seqcount.seqcount);
 	spin_unlock(&sl->lock);
 }
 
@@ -863,7 +873,7 @@ static inline void write_sequnlock(seqlock_t *sl)
 static inline void write_seqlock_bh(seqlock_t *sl)
 {
 	spin_lock_bh(&sl->lock);
-	write_seqcount_t_begin(&sl->seqcount);
+	write_seqcount_t_begin(&sl->seqcount.seqcount);
 }
 
 /**
@@ -876,7 +886,7 @@ static inline void write_seqlock_bh(seqlock_t *sl)
  */
 static inline void write_sequnlock_bh(seqlock_t *sl)
 {
-	write_seqcount_t_end(&sl->seqcount);
+	write_seqcount_t_end(&sl->seqcount.seqcount);
 	spin_unlock_bh(&sl->lock);
 }
 
@@ -890,7 +900,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
 static inline void write_seqlock_irq(seqlock_t *sl)
 {
 	spin_lock_irq(&sl->lock);
-	write_seqcount_t_begin(&sl->seqcount);
+	write_seqcount_t_begin(&sl->seqcount.seqcount);
 }
 
 /**
@@ -902,7 +912,7 @@ static inline void write_seqlock_irq(seqlock_t *sl)
  */
 static inline void write_sequnlock_irq(seqlock_t *sl)
 {
-	write_seqcount_t_end(&sl->seqcount);
+	write_seqcount_t_end(&sl->seqcount.seqcount);
 	spin_unlock_irq(&sl->lock);
 }
 
@@ -911,7 +921,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
 	unsigned long flags;
 
 	spin_lock_irqsave(&sl->lock, flags);
-	write_seqcount_t_begin(&sl->seqcount);
+	write_seqcount_t_begin(&sl->seqcount.seqcount);
 	return flags;
 }
 
@@ -940,7 +950,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
 static inline void
 write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
 {
-	write_seqcount_t_end(&sl->seqcount);
+	write_seqcount_t_end(&sl->seqcount.seqcount);
 	spin_unlock_irqrestore(&sl->lock, flags);
 }
 
-- 
2.28.0

Re: [PATCH v2 2/5] seqlock: Use unique prefix for seqcount_t property accessors

[peterz]

On Fri, Sep 04, 2020 at 05:32:28PM +0200, Ahmed S. Darwish wrote:

>  static __always_inline seqcount_t *					\
> -__seqcount_##lockname##_ptr(seqcount_##lockname##_t *s)			\
> +__seqprop_seqcount_##lockname##_ptr(seqcount_##lockname##_t *s)		\

I did s/__seqprop_seqcount_/__seqprop_/g on all this.

seqprop is a contraction of seqcount property, no need to put in yet
another seqcount I feel.

Re: [PATCH v2 4/5] seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support

[peterz]

On Fri, Sep 04, 2020 at 05:32:30PM +0200, Ahmed S. Darwish wrote:
> @@ -406,13 +443,20 @@ static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
>  	return __read_seqcount_t_retry(s, start);
>  }
>  
> +/*
> + * Enforce non-preemptibility for all seqcount_LOCKNAME_t writers. Don't
> + * do it for PREEMPT_RT, for the reasons outlined at __SEQ_LOCK().
> + */
> +#define __seq_enforce_writer_non_preemptibility(s)			\
> +	(!IS_ENABLED(CONFIG_PREEMPT_RT) && __seqcount_lock_preemptible(s))
> +
>  /**
>   * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
>   * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
>   */
>  #define raw_write_seqcount_begin(s)					\
>  do {									\
> -	if (__seqcount_lock_preemptible(s))				\
> +	if (__seq_enforce_writer_non_preemptibility(s))			\
>  		preempt_disable();					\
>  									\
>  	raw_write_seqcount_t_begin(__seqcount_ptr(s));			\
> @@ -433,7 +477,7 @@ static inline void raw_write_seqcount_t_begin(seqcount_t *s)
>  do {									\
>  	raw_write_seqcount_t_end(__seqcount_ptr(s));			\
>  									\
> -	if (__seqcount_lock_preemptible(s))				\
> +	if (__seq_enforce_writer_non_preemptibility(s))			\
>  		preempt_enable();					\
>  } while (0)
>  
> @@ -456,7 +500,7 @@ static inline void raw_write_seqcount_t_end(seqcount_t *s)
>  do {									\
>  	__seqcount_assert_lock_held(s);					\
>  									\
> -	if (__seqcount_lock_preemptible(s))				\
> +	if (__seq_enforce_writer_non_preemptibility(s))			\
>  		preempt_disable();					\
>  									\
>  	write_seqcount_t_begin_nested(__seqcount_ptr(s), subclass);	\
> @@ -483,7 +527,7 @@ static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
>  do {									\
>  	__seqcount_assert_lock_held(s);					\
>  									\
> -	if (__seqcount_lock_preemptible(s))				\
> +	if (__seq_enforce_writer_non_preemptibility(s))			\
>  		preempt_disable();					\
>  									\
>  	write_seqcount_t_begin(__seqcount_ptr(s));			\
> @@ -504,7 +548,7 @@ static inline void write_seqcount_t_begin(seqcount_t *s)
>  do {									\
>  	write_seqcount_t_end(__seqcount_ptr(s));			\
>  									\
> -	if (__seqcount_lock_preemptible(s))				\
> +	if (__seq_enforce_writer_non_preemptibility(s))			\
>  		preempt_enable();					\
>  } while (0)

I've replaced the above with the below, afaict there were no users of
__seqcount_lock_preemptible() left.

--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -228,7 +228,11 @@ __seqprop_##lockname##_sequence(const se
 static __always_inline bool						\
 __seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s)	\
 {									\
-	return preemptible;						\
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))				\
+		return preemptible;					\
+									\
+	/* PREEMPT_RT relies on the above LOCK+UNLOCK */		\
+	return false;							\
 }									\
 									\
 static __always_inline void						\

Re: [PATCH v2 4/5] seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support

[Ahmed S. Darwish]

On Tue, Sep 08, 2020 at 01:45:20PM +0200, peterz@infradead.org wrote:
> On Fri, Sep 04, 2020 at 05:32:30PM +0200, Ahmed S. Darwish wrote:
> > @@ -406,13 +443,20 @@ static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
> >  	return __read_seqcount_t_retry(s, start);
> >  }
> >
> > +/*
> > + * Enforce non-preemptibility for all seqcount_LOCKNAME_t writers. Don't
> > + * do it for PREEMPT_RT, for the reasons outlined at __SEQ_LOCK().
> > + */
> > +#define __seq_enforce_writer_non_preemptibility(s)			\
> > +	(!IS_ENABLED(CONFIG_PREEMPT_RT) && __seqcount_lock_preemptible(s))
> > +
> >  /**
> >   * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
> >   * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
> >   */
> >  #define raw_write_seqcount_begin(s)					\
> >  do {									\
> > -	if (__seqcount_lock_preemptible(s))				\
> > +	if (__seq_enforce_writer_non_preemptibility(s))			\
> >  		preempt_disable();					\
> >  									\
> >  	raw_write_seqcount_t_begin(__seqcount_ptr(s));			\
> > @@ -433,7 +477,7 @@ static inline void raw_write_seqcount_t_begin(seqcount_t *s)
> >  do {									\
> >  	raw_write_seqcount_t_end(__seqcount_ptr(s));			\
> >  									\
> > -	if (__seqcount_lock_preemptible(s))				\
> > +	if (__seq_enforce_writer_non_preemptibility(s))			\
> >  		preempt_enable();					\
> >  } while (0)
> >
> > @@ -456,7 +500,7 @@ static inline void raw_write_seqcount_t_end(seqcount_t *s)
> >  do {									\
> >  	__seqcount_assert_lock_held(s);					\
> >  									\
> > -	if (__seqcount_lock_preemptible(s))				\
> > +	if (__seq_enforce_writer_non_preemptibility(s))			\
> >  		preempt_disable();					\
> >  									\
> >  	write_seqcount_t_begin_nested(__seqcount_ptr(s), subclass);	\
> > @@ -483,7 +527,7 @@ static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
> >  do {									\
> >  	__seqcount_assert_lock_held(s);					\
> >  									\
> > -	if (__seqcount_lock_preemptible(s))				\
> > +	if (__seq_enforce_writer_non_preemptibility(s))			\
> >  		preempt_disable();					\
> >  									\
> >  	write_seqcount_t_begin(__seqcount_ptr(s));			\
> > @@ -504,7 +548,7 @@ static inline void write_seqcount_t_begin(seqcount_t *s)
> >  do {									\
> >  	write_seqcount_t_end(__seqcount_ptr(s));			\
> >  									\
> > -	if (__seqcount_lock_preemptible(s))				\
> > +	if (__seq_enforce_writer_non_preemptibility(s))			\
> >  		preempt_enable();					\
> >  } while (0)
>
> I've replaced the above with the below, afaict there were no users of
> __seqcount_lock_preemptible() left.
>
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -228,7 +228,11 @@ __seqprop_##lockname##_sequence(const se
>  static __always_inline bool						\
>  __seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s)	\
>  {									\
> -	return preemptible;						\
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))				\
> +		return preemptible;					\
> +									\
> +	/* PREEMPT_RT relies on the above LOCK+UNLOCK */		\
> +	return false;							\
>  }									\
>  									\

Sounds good.

[tip: locking/core] seqlock: PREEMPT_RT: Do not starve seqlock_t writers

[tip-bot2 for Ahmed S. Darwish]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     1909760f5fc3f123e47b4e24e0ccdc0fc8f3f106
Gitweb:        https://git.kernel.org/tip/1909760f5fc3f123e47b4e24e0ccdc0fc8f3f106
Author:        Ahmed S. Darwish <a.darwish@linutronix.de>
AuthorDate:    Fri, 04 Sep 2020 17:32:31 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 10 Sep 2020 11:19:31 +02:00

seqlock: PREEMPT_RT: Do not starve seqlock_t writers

On PREEMPT_RT, seqlock_t is transformed to a sleeping lock that do not
disable preemption. A seqlock_t reader can thus preempt its write side
section and spin for the enter scheduler tick. If that reader belongs to
a real-time scheduling class, it can spin forever and the kernel will
livelock.

To break this livelock possibility on PREEMPT_RT, implement seqlock_t in
terms of "seqcount_spinlock_t" instead of plain "seqcount_t".

Beside its pure annotational value, this will leverage the existing
seqcount_LOCKNAME_T PREEMPT_RT anti-livelock mechanisms, without adding
any extra code.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200904153231.11994-6-a.darwish@linutronix.de
---
 include/linux/seqlock.h | 32 +++++++++++++++++++++-----------
 1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 2bc9510..f73c7eb 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -790,13 +790,17 @@ static inline void raw_write_seqcount_latch(seqcount_latch_t *s)
  *    - Documentation/locking/seqlock.rst
  */
 typedef struct {
-	struct seqcount seqcount;
+	/*
+	 * Make sure that readers don't starve writers on PREEMPT_RT: use
+	 * seqcount_spinlock_t instead of seqcount_t. Check __SEQ_LOCK().
+	 */
+	seqcount_spinlock_t seqcount;
 	spinlock_t lock;
 } seqlock_t;
 
 #define __SEQLOCK_UNLOCKED(lockname)					\
 	{								\
-		.seqcount = SEQCNT_ZERO(lockname),			\
+		.seqcount = SEQCNT_SPINLOCK_ZERO(lockname, &(lockname).lock), \
 		.lock =	__SPIN_LOCK_UNLOCKED(lockname)			\
 	}
 
@@ -806,8 +810,8 @@ typedef struct {
  */
 #define seqlock_init(sl)						\
 	do {								\
-		seqcount_init(&(sl)->seqcount);				\
 		spin_lock_init(&(sl)->lock);				\
+		seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock);	\
 	} while (0)
 
 /**
@@ -854,6 +858,12 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
 	return read_seqcount_retry(&sl->seqcount, start);
 }
 
+/*
+ * For all seqlock_t write side functions, use write_seqcount_*t*_begin()
+ * instead of the generic write_seqcount_begin(). This way, no redundant
+ * lockdep_assert_held() checks are added.
+ */
+
 /**
  * write_seqlock() - start a seqlock_t write side critical section
  * @sl: Pointer to seqlock_t
@@ -870,7 +880,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
 static inline void write_seqlock(seqlock_t *sl)
 {
 	spin_lock(&sl->lock);
-	write_seqcount_t_begin(&sl->seqcount);
+	write_seqcount_t_begin(&sl->seqcount.seqcount);
 }
 
 /**
@@ -882,7 +892,7 @@ static inline void write_seqlock(seqlock_t *sl)
  */
 static inline void write_sequnlock(seqlock_t *sl)
 {
-	write_seqcount_t_end(&sl->seqcount);
+	write_seqcount_t_end(&sl->seqcount.seqcount);
 	spin_unlock(&sl->lock);
 }
 
@@ -896,7 +906,7 @@ static inline void write_sequnlock(seqlock_t *sl)
 static inline void write_seqlock_bh(seqlock_t *sl)
 {
 	spin_lock_bh(&sl->lock);
-	write_seqcount_t_begin(&sl->seqcount);
+	write_seqcount_t_begin(&sl->seqcount.seqcount);
 }
 
 /**
@@ -909,7 +919,7 @@ static inline void write_seqlock_bh(seqlock_t *sl)
  */
 static inline void write_sequnlock_bh(seqlock_t *sl)
 {
-	write_seqcount_t_end(&sl->seqcount);
+	write_seqcount_t_end(&sl->seqcount.seqcount);
 	spin_unlock_bh(&sl->lock);
 }
 
@@ -923,7 +933,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
 static inline void write_seqlock_irq(seqlock_t *sl)
 {
 	spin_lock_irq(&sl->lock);
-	write_seqcount_t_begin(&sl->seqcount);
+	write_seqcount_t_begin(&sl->seqcount.seqcount);
 }
 
 /**
@@ -935,7 +945,7 @@ static inline void write_seqlock_irq(seqlock_t *sl)
  */
 static inline void write_sequnlock_irq(seqlock_t *sl)
 {
-	write_seqcount_t_end(&sl->seqcount);
+	write_seqcount_t_end(&sl->seqcount.seqcount);
 	spin_unlock_irq(&sl->lock);
 }
 
@@ -944,7 +954,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
 	unsigned long flags;
 
 	spin_lock_irqsave(&sl->lock, flags);
-	write_seqcount_t_begin(&sl->seqcount);
+	write_seqcount_t_begin(&sl->seqcount.seqcount);
 	return flags;
 }
 
@@ -973,7 +983,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
 static inline void
 write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
 {
-	write_seqcount_t_end(&sl->seqcount);
+	write_seqcount_t_end(&sl->seqcount.seqcount);
 	spin_unlock_irqrestore(&sl->lock, flags);
 }
 

[tip: locking/core] seqlock: seqcount_t: Implement all read APIs as statement expressions

[tip-bot2 for Ahmed S. Darwish]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     52ac39e5db5148f70392edb654ad882ac8da88a8
Gitweb:        https://git.kernel.org/tip/52ac39e5db5148f70392edb654ad882ac8da88a8
Author:        Ahmed S. Darwish <a.darwish@linutronix.de>
AuthorDate:    Fri, 04 Sep 2020 17:32:29 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 10 Sep 2020 11:19:31 +02:00

seqlock: seqcount_t: Implement all read APIs as statement expressions

The sequence counters read APIs are implemented as CPP macros, so they
can take either seqcount_t or any of the seqcount_LOCKNAME_t variants.
Such macros then get *directly* transformed to internal C functions that
only take plain seqcount_t.

Further commits need access to seqcount_LOCKNAME_t inside of the actual
read APIs code. Thus transform all of the seqcount read APIs to pure GCC
statement expressions instead.

This will not break type-safety: all of the transformed APIs resolve to
a _Generic() selection that does not have a "default" case.

This will also not affect the transformed APIs readability: previously
added kernel-doc above all of seqlock.h functions makes the expectations
quite clear for call-site developers.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200904153231.11994-4-a.darwish@linutronix.de
---
 include/linux/seqlock.h | 94 +++++++++++++++++++---------------------
 1 file changed, 45 insertions(+), 49 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 0b4a22f..f3b7827 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -184,6 +184,12 @@ __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s)			\
 	return &s->seqcount;						\
 }									\
 									\
+static __always_inline unsigned						\
+__seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s)	\
+{									\
+	return READ_ONCE(s->seqcount.sequence);				\
+}									\
+									\
 static __always_inline bool						\
 __seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s)	\
 {									\
@@ -205,6 +211,11 @@ static inline seqcount_t *__seqprop_ptr(seqcount_t *s)
 	return s;
 }
 
+static inline unsigned __seqprop_sequence(const seqcount_t *s)
+{
+	return READ_ONCE(s->sequence);
+}
+
 static inline bool __seqprop_preemptible(const seqcount_t *s)
 {
 	return false;
@@ -250,6 +261,7 @@ SEQCOUNT_LOCKNAME(ww_mutex,	struct ww_mutex,	true,	&s->lock->base)
 	__seqprop_case((s),	ww_mutex,	prop))
 
 #define __seqcount_ptr(s)		__seqprop(s, ptr)
+#define __seqcount_sequence(s)		__seqprop(s, sequence)
 #define __seqcount_lock_preemptible(s)	__seqprop(s, preemptible)
 #define __seqcount_assert_lock_held(s)	__seqprop(s, assert)
 
@@ -268,21 +280,15 @@ SEQCOUNT_LOCKNAME(ww_mutex,	struct ww_mutex,	true,	&s->lock->base)
  * Return: count to be passed to read_seqcount_retry()
  */
 #define __read_seqcount_begin(s)					\
-	__read_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
-{
-	unsigned ret;
-
-repeat:
-	ret = READ_ONCE(s->sequence);
-	if (unlikely(ret & 1)) {
-		cpu_relax();
-		goto repeat;
-	}
-	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
-	return ret;
-}
+({									\
+	unsigned seq;							\
+									\
+	while ((seq = __seqcount_sequence(s)) & 1)			\
+		cpu_relax();						\
+									\
+	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);			\
+	seq;								\
+})
 
 /**
  * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
@@ -291,14 +297,12 @@ repeat:
  * Return: count to be passed to read_seqcount_retry()
  */
 #define raw_read_seqcount_begin(s)					\
-	raw_read_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
-{
-	unsigned ret = __read_seqcount_t_begin(s);
-	smp_rmb();
-	return ret;
-}
+({									\
+	unsigned seq = __read_seqcount_begin(s);			\
+									\
+	smp_rmb();							\
+	seq;								\
+})
 
 /**
  * read_seqcount_begin() - begin a seqcount_t read critical section
@@ -307,13 +311,10 @@ static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
  * Return: count to be passed to read_seqcount_retry()
  */
 #define read_seqcount_begin(s)						\
-	read_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
-{
-	seqcount_lockdep_reader_access(s);
-	return raw_read_seqcount_t_begin(s);
-}
+({									\
+	seqcount_lockdep_reader_access(__seqcount_ptr(s));		\
+	raw_read_seqcount_begin(s);					\
+})
 
 /**
  * raw_read_seqcount() - read the raw seqcount_t counter value
@@ -327,15 +328,13 @@ static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
  * Return: count to be passed to read_seqcount_retry()
  */
 #define raw_read_seqcount(s)						\
-	raw_read_seqcount_t(__seqcount_ptr(s))
-
-static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
-{
-	unsigned ret = READ_ONCE(s->sequence);
-	smp_rmb();
-	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
-	return ret;
-}
+({									\
+	unsigned seq = __seqcount_sequence(s);				\
+									\
+	smp_rmb();							\
+	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);			\
+	seq;								\
+})
 
 /**
  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
@@ -355,16 +354,13 @@ static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
  * Return: count to be passed to read_seqcount_retry()
  */
 #define raw_seqcount_begin(s)						\
-	raw_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
-{
-	/*
-	 * If the counter is odd, let read_seqcount_retry() fail
-	 * by decrementing the counter.
-	 */
-	return raw_read_seqcount_t(s) & ~1;
-}
+({									\
+	/*								\
+	 * If the counter is odd, let read_seqcount_retry() fail	\
+	 * by decrementing the counter.					\
+	 */								\
+	raw_read_seqcount(s) & ~1;					\
+})
 
 /**
  * __read_seqcount_retry() - end a seqcount_t read section w/o barrier

[tip: locking/core] seqlock: Use unique prefix for seqcount_t property accessors

[tip-bot2 for Ahmed S. Darwish]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     5cdd25572a29e46f932d3e6eedbd07429de66431
Gitweb:        https://git.kernel.org/tip/5cdd25572a29e46f932d3e6eedbd07429de66431
Author:        Ahmed S. Darwish <a.darwish@linutronix.de>
AuthorDate:    Fri, 04 Sep 2020 17:32:28 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 10 Sep 2020 11:19:30 +02:00

seqlock: Use unique prefix for seqcount_t property accessors

At seqlock.h, the following set of functions:

    - __seqcount_ptr()
    - __seqcount_preemptible()
    - __seqcount_assert()

act as plain seqcount_t "property" accessors. Meanwhile, the following
group:

    - __seqcount_ptr()
    - __seqcount_lock_preemptible()
    - __seqcount_assert_lock_held()

act as the equivalent set, but in the generic form, taking either
seqcount_t or any of the seqcount_LOCKNAME_t variants.

This is quite confusing, especially the first member where it is called
exactly the same in both groups.

Differentiate the first group by using "__seqprop" as prefix, and also
use that same prefix for all of seqcount_LOCKNAME_t property accessors.

While at it, constify the property accessors first parameter when
appropriate.

References: 55f3560df975 ("seqlock: Extend seqcount API with associated locks")
Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200904153231.11994-3-a.darwish@linutronix.de
---
 include/linux/seqlock.h | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 820ace2..0b4a22f 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -157,7 +157,9 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
  */
 
 /*
- * SEQCOUNT_LOCKNAME() - Instantiate seqcount_LOCKNAME_t and helpers
+ * SEQCOUNT_LOCKNAME()	- Instantiate seqcount_LOCKNAME_t and helpers
+ * seqprop_LOCKNAME_*()	- Property accessors for seqcount_LOCKNAME_t
+ *
  * @lockname:		"LOCKNAME" part of seqcount_LOCKNAME_t
  * @locktype:		LOCKNAME canonical C data type
  * @preemptible:	preemptibility of above lockname
@@ -177,19 +179,19 @@ seqcount_##lockname##_init(seqcount_##lockname##_t *s, locktype *lock)	\
 }									\
 									\
 static __always_inline seqcount_t *					\
-__seqcount_##lockname##_ptr(seqcount_##lockname##_t *s)			\
+__seqprop_##lockname##_ptr(seqcount_##lockname##_t *s)			\
 {									\
 	return &s->seqcount;						\
 }									\
 									\
 static __always_inline bool						\
-__seqcount_##lockname##_preemptible(seqcount_##lockname##_t *s)		\
+__seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s)	\
 {									\
 	return preemptible;						\
 }									\
 									\
 static __always_inline void						\
-__seqcount_##lockname##_assert(seqcount_##lockname##_t *s)		\
+__seqprop_##lockname##_assert(const seqcount_##lockname##_t *s)		\
 {									\
 	__SEQ_LOCK(lockdep_assert_held(lockmember));			\
 }
@@ -198,17 +200,17 @@ __seqcount_##lockname##_assert(seqcount_##lockname##_t *s)		\
  * __seqprop() for seqcount_t
  */
 
-static inline seqcount_t *__seqcount_ptr(seqcount_t *s)
+static inline seqcount_t *__seqprop_ptr(seqcount_t *s)
 {
 	return s;
 }
 
-static inline bool __seqcount_preemptible(seqcount_t *s)
+static inline bool __seqprop_preemptible(const seqcount_t *s)
 {
 	return false;
 }
 
-static inline void __seqcount_assert(seqcount_t *s)
+static inline void __seqprop_assert(const seqcount_t *s)
 {
 	lockdep_assert_preemption_disabled();
 }
@@ -237,10 +239,10 @@ SEQCOUNT_LOCKNAME(ww_mutex,	struct ww_mutex,	true,	&s->lock->base)
 #define SEQCNT_WW_MUTEX_ZERO(name, lock) 	SEQCOUNT_LOCKNAME_ZERO(name, lock)
 
 #define __seqprop_case(s, lockname, prop)				\
-	seqcount_##lockname##_t: __seqcount_##lockname##_##prop((void *)(s))
+	seqcount_##lockname##_t: __seqprop_##lockname##_##prop((void *)(s))
 
 #define __seqprop(s, prop) _Generic(*(s),				\
-	seqcount_t:		__seqcount_##prop((void *)(s)),		\
+	seqcount_t:		__seqprop_##prop((void *)(s)),		\
 	__seqprop_case((s),	raw_spinlock,	prop),			\
 	__seqprop_case((s),	spinlock,	prop),			\
 	__seqprop_case((s),	rwlock,		prop),			\

[tip: locking/core] seqlock: seqcount_LOCKNAME_t: Standardize naming convention

[tip-bot2 for Ahmed S. Darwish]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     6dd699b13d53f26a7603702d8bada3482312df74
Gitweb:        https://git.kernel.org/tip/6dd699b13d53f26a7603702d8bada3482312df74
Author:        Ahmed S. Darwish <a.darwish@linutronix.de>
AuthorDate:    Fri, 04 Sep 2020 17:32:27 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 10 Sep 2020 11:19:30 +02:00

seqlock: seqcount_LOCKNAME_t: Standardize naming convention

At seqlock.h, sequence counters with associated locks are either called
seqcount_LOCKNAME_t, seqcount_LOCKTYPE_t, or seqcount_locktype_t.

Standardize on seqcount_LOCKNAME_t for all instances in comments,
kernel-doc, and SEQCOUNT_LOCKNAME() generative macro paramters.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200904153231.11994-2-a.darwish@linutronix.de
---
 include/linux/seqlock.h | 79 ++++++++++++++++++++--------------------
 1 file changed, 40 insertions(+), 39 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index f2a7a46..820ace2 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -53,7 +53,7 @@
  *
  * If the write serialization mechanism is one of the common kernel
  * locking primitives, use a sequence counter with associated lock
- * (seqcount_LOCKTYPE_t) instead.
+ * (seqcount_LOCKNAME_t) instead.
  *
  * If it's desired to automatically handle the sequence counter writer
  * serialization and non-preemptibility requirements, use a sequential
@@ -117,7 +117,7 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
 #define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
 
 /*
- * Sequence counters with associated locks (seqcount_LOCKTYPE_t)
+ * Sequence counters with associated locks (seqcount_LOCKNAME_t)
  *
  * A sequence counter which associates the lock used for writer
  * serialization at initialization time. This enables lockdep to validate
@@ -138,30 +138,32 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
 #endif
 
 /**
- * typedef seqcount_LOCKNAME_t - sequence counter with LOCKTYPE associated
+ * typedef seqcount_LOCKNAME_t - sequence counter with LOCKNAME associated
  * @seqcount:	The real sequence counter
- * @lock:	Pointer to the associated spinlock
+ * @lock:	Pointer to the associated lock
  *
- * A plain sequence counter with external writer synchronization by a
- * spinlock. The spinlock is associated to the sequence count in the
+ * A plain sequence counter with external writer synchronization by
+ * LOCKNAME @lock. The lock is associated to the sequence counter in the
  * static initializer or init function. This enables lockdep to validate
  * that the write side critical section is properly serialized.
+ *
+ * LOCKNAME:	raw_spinlock, spinlock, rwlock, mutex, or ww_mutex.
  */
 
 /*
  * seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t
  * @s:		Pointer to the seqcount_LOCKNAME_t instance
- * @lock:	Pointer to the associated LOCKTYPE
+ * @lock:	Pointer to the associated lock
  */
 
 /*
- * SEQCOUNT_LOCKTYPE() - Instantiate seqcount_LOCKNAME_t and helpers
- * @locktype:		actual typename
- * @lockname:		name
- * @preemptible:	preemptibility of above locktype
+ * SEQCOUNT_LOCKNAME() - Instantiate seqcount_LOCKNAME_t and helpers
+ * @lockname:		"LOCKNAME" part of seqcount_LOCKNAME_t
+ * @locktype:		LOCKNAME canonical C data type
+ * @preemptible:	preemptibility of above lockname
  * @lockmember:		argument for lockdep_assert_held()
  */
-#define SEQCOUNT_LOCKTYPE(locktype, lockname, preemptible, lockmember)	\
+#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember)	\
 typedef struct seqcount_##lockname {					\
 	seqcount_t		seqcount;				\
 	__SEQ_LOCK(locktype	*lock);					\
@@ -211,29 +213,28 @@ static inline void __seqcount_assert(seqcount_t *s)
 	lockdep_assert_preemption_disabled();
 }
 
-SEQCOUNT_LOCKTYPE(raw_spinlock_t,	raw_spinlock,	false,	s->lock)
-SEQCOUNT_LOCKTYPE(spinlock_t,		spinlock,	false,	s->lock)
-SEQCOUNT_LOCKTYPE(rwlock_t,		rwlock,		false,	s->lock)
-SEQCOUNT_LOCKTYPE(struct mutex,		mutex,		true,	s->lock)
-SEQCOUNT_LOCKTYPE(struct ww_mutex,	ww_mutex,	true,	&s->lock->base)
+SEQCOUNT_LOCKNAME(raw_spinlock,	raw_spinlock_t,		false,	s->lock)
+SEQCOUNT_LOCKNAME(spinlock,	spinlock_t,		false,	s->lock)
+SEQCOUNT_LOCKNAME(rwlock,	rwlock_t,		false,	s->lock)
+SEQCOUNT_LOCKNAME(mutex,	struct mutex,		true,	s->lock)
+SEQCOUNT_LOCKNAME(ww_mutex,	struct ww_mutex,	true,	&s->lock->base)
 
 /*
  * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
  * @name:	Name of the seqcount_LOCKNAME_t instance
- * @lock:	Pointer to the associated LOCKTYPE
+ * @lock:	Pointer to the associated LOCKNAME
  */
 
-#define SEQCOUNT_LOCKTYPE_ZERO(seq_name, assoc_lock) {			\
+#define SEQCOUNT_LOCKNAME_ZERO(seq_name, assoc_lock) {			\
 	.seqcount		= SEQCNT_ZERO(seq_name.seqcount),	\
 	__SEQ_LOCK(.lock	= (assoc_lock))				\
 }
 
-#define SEQCNT_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKTYPE_ZERO(name, lock)
-#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKTYPE_ZERO(name, lock)
-#define SEQCNT_RWLOCK_ZERO(name, lock)		SEQCOUNT_LOCKTYPE_ZERO(name, lock)
-#define SEQCNT_MUTEX_ZERO(name, lock)		SEQCOUNT_LOCKTYPE_ZERO(name, lock)
-#define SEQCNT_WW_MUTEX_ZERO(name, lock) 	SEQCOUNT_LOCKTYPE_ZERO(name, lock)
-
+#define SEQCNT_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name, lock)
+#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name, lock)
+#define SEQCNT_RWLOCK_ZERO(name, lock)		SEQCOUNT_LOCKNAME_ZERO(name, lock)
+#define SEQCNT_MUTEX_ZERO(name, lock)		SEQCOUNT_LOCKNAME_ZERO(name, lock)
+#define SEQCNT_WW_MUTEX_ZERO(name, lock) 	SEQCOUNT_LOCKNAME_ZERO(name, lock)
 
 #define __seqprop_case(s, lockname, prop)				\
 	seqcount_##lockname##_t: __seqcount_##lockname##_##prop((void *)(s))
@@ -252,7 +253,7 @@ SEQCOUNT_LOCKTYPE(struct ww_mutex,	ww_mutex,	true,	&s->lock->base)
 
 /**
  * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -283,7 +284,7 @@ repeat:
 
 /**
  * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * Return: count to be passed to read_seqcount_retry()
  */
@@ -299,7 +300,7 @@ static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
 
 /**
  * read_seqcount_begin() - begin a seqcount_t read critical section
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * Return: count to be passed to read_seqcount_retry()
  */
@@ -314,7 +315,7 @@ static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
 
 /**
  * raw_read_seqcount() - read the raw seqcount_t counter value
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * raw_read_seqcount opens a read critical section of the given
  * seqcount_t, without any lockdep checking, and without checking or
@@ -337,7 +338,7 @@ static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
 /**
  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
  *                        lockdep and w/o counter stabilization
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * raw_seqcount_begin opens a read critical section of the given
  * seqcount_t. Unlike read_seqcount_begin(), this function will not wait
@@ -365,7 +366,7 @@ static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
 
 /**
  * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  * @start: count, from read_seqcount_begin()
  *
  * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
@@ -389,7 +390,7 @@ static inline int __read_seqcount_t_retry(const seqcount_t *s, unsigned start)
 
 /**
  * read_seqcount_retry() - end a seqcount_t read critical section
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  * @start: count, from read_seqcount_begin()
  *
  * read_seqcount_retry closes the read critical section of given
@@ -409,7 +410,7 @@ static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
 
 /**
  * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  */
 #define raw_write_seqcount_begin(s)					\
 do {									\
@@ -428,7 +429,7 @@ static inline void raw_write_seqcount_t_begin(seqcount_t *s)
 
 /**
  * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  */
 #define raw_write_seqcount_end(s)					\
 do {									\
@@ -448,7 +449,7 @@ static inline void raw_write_seqcount_t_end(seqcount_t *s)
 /**
  * write_seqcount_begin_nested() - start a seqcount_t write section with
  *                                 custom lockdep nesting level
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  * @subclass: lockdep nesting level
  *
  * See Documentation/locking/lockdep-design.rst
@@ -471,7 +472,7 @@ static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
 
 /**
  * write_seqcount_begin() - start a seqcount_t write side critical section
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * write_seqcount_begin opens a write side critical section of the given
  * seqcount_t.
@@ -497,7 +498,7 @@ static inline void write_seqcount_t_begin(seqcount_t *s)
 
 /**
  * write_seqcount_end() - end a seqcount_t write side critical section
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * The write section must've been opened with write_seqcount_begin().
  */
@@ -517,7 +518,7 @@ static inline void write_seqcount_t_end(seqcount_t *s)
 
 /**
  * raw_write_seqcount_barrier() - do a seqcount_t write barrier
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * This can be used to provide an ordering guarantee instead of the usual
  * consistency guarantee. It is one wmb cheaper, because it can collapse
@@ -571,7 +572,7 @@ static inline void raw_write_seqcount_t_barrier(seqcount_t *s)
 /**
  * write_seqcount_invalidate() - invalidate in-progress seqcount_t read
  *                               side operations
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * After write_seqcount_invalidate, no seqcount_t read side operations
  * will complete successfully and see data older than this.

Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

[Qian Cai]

On Fri, 2020-09-04 at 17:32 +0200, Ahmed S. Darwish wrote:
> Hi,
> 
> Changelog-v2
> ============
> 
>   - Standardize on seqcount_LOCKNAME_t as the canonical reference for
>     sequence counters with associated locks, instead of v1
>     seqcount_LOCKTYPE_t.
> 
>   - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t
>     property accessors.
> 
>   - Touch-up the lock-unlock rationale for more clarity. Enforce writer
>     non-preemitiblity using "__seq_enforce_writer_non_preemptibility()".
> 
> Cover letter (v1)
> =================
> 
> https://lkml.kernel.org/r/20200828010710.5407-1-a.darwish@linutronix.de
> 
> Preemption must be disabled before entering a sequence counter write
> side critical section.  Otherwise the read side section can preempt the
> write side section and spin for the entire scheduler tick.  If that
> reader belongs to a real-time scheduling class, it can spin forever and
> the kernel will livelock.
> 
> Disabling preemption cannot be done for PREEMPT_RT though: it can lead
> to higher latencies, and the write side sections will not be able to
> acquire locks which become sleeping locks (e.g. spinlock_t).
> 
> To remain preemptible, while avoiding a possible livelock caused by the
> reader preempting the writer, use a different technique: let the reader
> detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
> case, acquire then release the associated LOCKNAME writer serialization
> lock. This will allow any possibly-preempted writer to make progress
> until the end of its writer serialization lock critical section.
> 
> Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
> an associated (PREEMPT_RT) sleeping lock, and for seqlock_t.

Reverting this patchset [1] from today's linux-next fixed a splat below. The
splat looks like a false positive anyway because the existing locking dependency
chains from the task #1 here:

&s->seqcount#2 ---> pidmap_lock

[  528.078061][ T7867] -> #1 (pidmap_lock){....}-{2:2}:
[  528.078078][ T7867]        lock_acquire+0x10c/0x560
[  528.078089][ T7867]        _raw_spin_lock_irqsave+0x64/0xb0
[  528.078108][ T7867]        free_pid+0x5c/0x160
free_pid at kernel/pid.c:131
[  528.078127][ T7867]        release_task.part.40+0x59c/0x7f0
__unhash_process at kernel/exit.c:76
(inlined by) __exit_signal at kernel/exit.c:147
(inlined by) release_task at kernel/exit.c:198
[  528.078145][ T7867]        do_exit+0x77c/0xda0
exit_notify at kernel/exit.c:679
(inlined by) do_exit at kernel/exit.c:826
[  528.078163][ T7867]        kthread+0x148/0x1d0
[  528.078182][ T7867]        ret_from_kernel_thread+0x5c/0x80

It is write_seqlock(&sig->stats_lock) in __exit_signal(), but the &s->seqcount#2 
in read_mems_allowed_begin() is read_seqcount_begin(&current->mems_allowed_seq), 
so there should be no deadlock?

[1] git revert --no-edit 0c9794c8b678..1909760f5fc3

[  528.077900][ T7867] WARNING: possible circular locking dependency detected
[  528.077912][ T7867] 5.9.0-rc5-next-20200914 #1 Not tainted
[  528.077921][ T7867] ------------------------------------------------------
[  528.077931][ T7867] runc:[1:CHILD]/7867 is trying to acquire lock:
[  528.077942][ T7867] c000001fce5570c8 (&s->seqcount#2){....}-{0:0}, at: __slab_alloc+0x34/0xf0
[  528.077972][ T7867] 
[  528.077972][ T7867] but task is already holding lock:
[  528.077983][ T7867] c0000000056b0198 (pidmap_lock){....}-{2:2}, at: alloc_pid+0x258/0x590
[  528.078009][ T7867] 
[  528.078009][ T7867] which lock already depends on the new lock.
[  528.078009][ T7867] 
[  528.078031][ T7867] 
[  528.078031][ T7867] the existing dependency chain (in reverse order) is:
[  528.078061][ T7867] 
[  528.078061][ T7867] -> #1 (pidmap_lock){....}-{2:2}:
[  528.078078][ T7867]        lock_acquire+0x10c/0x560
[  528.078089][ T7867]        _raw_spin_lock_irqsave+0x64/0xb0
[  528.078108][ T7867]        free_pid+0x5c/0x160
free_pid at kernel/pid.c:131
[  528.078127][ T7867]        release_task.part.40+0x59c/0x7f0
__unhash_process at kernel/exit.c:76
(inlined by) __exit_signal at kernel/exit.c:147
(inlined by) release_task at kernel/exit.c:198
[  528.078145][ T7867]        do_exit+0x77c/0xda0
exit_notify at kernel/exit.c:679
(inlined by) do_exit at kernel/exit.c:826
[  528.078163][ T7867]        kthread+0x148/0x1d0
[  528.078182][ T7867]        ret_from_kernel_thread+0x5c/0x80
[  528.078208][ T7867] 
[  528.078208][ T7867] -> #0 (&s->seqcount#2){....}-{0:0}:
[  528.078241][ T7867]        check_prevs_add+0x1c4/0x1120
check_prev_add at kernel/locking/lockdep.c:2820
(inlined by) check_prevs_add at kernel/locking/lockdep.c:2944
[  528.078260][ T7867]        __lock_acquire+0x176c/0x1c00
validate_chain at kernel/locking/lockdep.c:3562
(inlined by) __lock_acquire at kernel/locking/lockdep.c:4796
[  528.078278][ T7867]        lock_acquire+0x10c/0x560
[  528.078297][ T7867]        ___slab_alloc+0xa40/0xb40
seqcount_lockdep_reader_access at include/linux/seqlock.h:103
(inlined by) read_mems_allowed_begin at include/linux/cpuset.h:135
(inlined by) get_any_partial at mm/slub.c:2035
(inlined by) get_partial at mm/slub.c:2078
(inlined by) new_slab_objects at mm/slub.c:2577
(inlined by) ___slab_alloc at mm/slub.c:2745
[  528.078324][ T7867]        __slab_alloc+0x34/0xf0
[  528.078342][ T7867]        kmem_cache_alloc+0x2d4/0x470
[  528.078362][ T7867]        create_object+0x74/0x430
[  528.078381][ T7867]        slab_post_alloc_hook+0xa4/0x670
[  528.078399][ T7867]        kmem_cache_alloc+0x1b4/0x470
[  528.078418][ T7867]        radix_tree_node_alloc.constprop.19+0xe4/0x160
[  528.078438][ T7867]        idr_get_free+0x298/0x360
[  528.078456][ T7867]        idr_alloc_u32+0x84/0x130
[  528.078474][ T7867]        idr_alloc_cyclic+0x7c/0x150
[  528.078493][ T7867]        alloc_pid+0x27c/0x590
[  528.078511][ T7867]        copy_process+0xc90/0x1930
copy_process at kernel/fork.c:2104
[  528.078529][ T7867]        kernel_clone+0x120/0xa10
[  528.078546][ T7867]        __do_sys_clone+0x88/0xd0
[  528.078565][ T7867]        system_call_exception+0xf8/0x1d0
[  528.078592][ T7867]        system_call_common+0xe8/0x218
[  528.078609][ T7867] 
[  528.078609][ T7867] other info that might help us debug this:
[  528.078609][ T7867] 
[  528.078650][ T7867]  Possible unsafe locking scenario:
[  528.078650][ T7867] 
[  528.078670][ T7867]        CPU0                    CPU1
[  528.078695][ T7867]        ----                    ----
[  528.078713][ T7867]   lock(pidmap_lock);
[  528.078730][ T7867]                                lock(&s->seqcount#2);
[  528.078751][ T7867]                                lock(pidmap_lock);
[  528.078770][ T7867]   lock(&s->seqcount#2);
[  528.078788][ T7867] 
[  528.078788][ T7867]  *** DEADLOCK ***
[  528.078788][ T7867] 
[  528.078800][ T7867] 2 locks held by runc:[1:CHILD]/7867:
[  528.078808][ T7867]  #0: c000001ffea6f4f0 (lock#2){+.+.}-{2:2}, at: __radix_tree_preload+0x8/0x370
__radix_tree_preload at lib/radix-tree.c:322
[  528.078844][ T7867]  #1: c0000000056b0198 (pidmap_lock){....}-{2:2}, at: alloc_pid+0x258/0x590
[  528.078870][ T7867] 
[  528.078870][ T7867] stack backtrace:
[  528.078890][ T7867] CPU: 46 PID: 7867 Comm: runc:[1:CHILD] Not tainted 5.9.0-rc5-next-20200914 #1
[  528.078921][ T7867] Call Trace:
[  528.078940][ T7867] [c000001ff07eefc0] [c00000000063f8c8] dump_stack+0xec/0x144 (unreliable)
[  528.078964][ T7867] [c000001ff07ef000] [c00000000013f44c] print_circular_bug.isra.43+0x2dc/0x350
[  528.078978][ T7867] [c000001ff07ef0a0] [c00000000013f640] check_noncircular+0x180/0x1b0
[  528.079000][ T7867] [c000001ff07ef170] [c000000000140b84] check_prevs_add+0x1c4/0x1120
[  528.079022][ T7867] [c000001ff07ef280] [c0000000001446ec] __lock_acquire+0x176c/0x1c00
[  528.079043][ T7867] [c000001ff07ef3a0] [c00000000014578c] lock_acquire+0x10c/0x560
[  528.079066][ T7867] [c000001ff07ef490] [c0000000003565f0] ___slab_alloc+0xa40/0xb40
[  528.079079][ T7867] [c000001ff07ef590] [c000000000356724] __slab_alloc+0x34/0xf0
[  528.079100][ T7867] [c000001ff07ef5e0] [c000000000356ab4] kmem_cache_alloc+0x2d4/0x470
[  528.079122][ T7867] [c000001ff07ef670] [c000000000397e14] create_object+0x74/0x430
[  528.079144][ T7867] [c000001ff07ef720] [c000000000351944] slab_post_alloc_hook+0xa4/0x670
[  528.079165][ T7867] [c000001ff07ef7e0] [c000000000356994] kmem_cache_alloc+0x1b4/0x470
[  528.079187][ T7867] [c000001ff07ef870] [c00000000064e004] radix_tree_node_alloc.constprop.19+0xe4/0x160
radix_tree_node_alloc at lib/radix-tree.c:252
[  528.079219][ T7867] [c000001ff07ef8e0] [c00000000064f2b8] idr_get_free+0x298/0x360
idr_get_free at lib/radix-tree.c:1507
[  528.079249][ T7867] [c000001ff07ef970] [c000000000645db4] idr_alloc_u32+0x84/0x130
idr_alloc_u32 at lib/idr.c:46 (discriminator 4)
[  528.079271][ T7867] [c000001ff07ef9e0] [c000000000645f8c] idr_alloc_cyclic+0x7c/0x150
idr_alloc_cyclic at lib/idr.c:126 (discriminator 1)
[  528.079301][ T7867] [c000001ff07efa40] [c0000000000e48ac] alloc_pid+0x27c/0x590
[  528.079342][ T7867] [c000001ff07efb20] [c0000000000acc60] copy_process+0xc90/0x1930
[  528.079404][ T7867] [c000001ff07efc40] [c0000000000adc00] kernel_clone+0x120/0xa10
[  528.079499][ T7867] [c000001ff07efd00] [c0000000000ae578] __do_sys_clone+0x88/0xd0
[  528.079579][ T7867] [c000001ff07efdc0] [c000000000029c48] system_call_exception+0xf8/0x1d0
[  528.079691][ T7867] [c000001ff07efe20] [c00000000000d0a8] system_call_common+0xe8/0x218

> 
> 8<--------------
> 
> Ahmed S. Darwish (5):
>   seqlock: seqcount_LOCKNAME_t: Standardize naming convention
>   seqlock: Use unique prefix for seqcount_t property accessors
>   seqlock: seqcount_t: Implement all read APIs as statement expressions
>   seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support
>   seqlock: PREEMPT_RT: Do not starve seqlock_t writers
> 
>  include/linux/seqlock.h | 281 ++++++++++++++++++++++++----------------
>  1 file changed, 167 insertions(+), 114 deletions(-)
> 
> base-commit: f75aef392f869018f78cfedf3c320a6b3fcfda6b
> --
> 2.28.0

Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

[Boqun Feng]

On Mon, Sep 14, 2020 at 08:20:53PM -0400, Qian Cai wrote:
> On Fri, 2020-09-04 at 17:32 +0200, Ahmed S. Darwish wrote:
> > Hi,
> > 
> > Changelog-v2
> > ============
> > 
> >   - Standardize on seqcount_LOCKNAME_t as the canonical reference for
> >     sequence counters with associated locks, instead of v1
> >     seqcount_LOCKTYPE_t.
> > 
> >   - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t
> >     property accessors.
> > 
> >   - Touch-up the lock-unlock rationale for more clarity. Enforce writer
> >     non-preemitiblity using "__seq_enforce_writer_non_preemptibility()".
> > 
> > Cover letter (v1)
> > =================
> > 
> > https://lkml.kernel.org/r/20200828010710.5407-1-a.darwish@linutronix.de
> > 
> > Preemption must be disabled before entering a sequence counter write
> > side critical section.  Otherwise the read side section can preempt the
> > write side section and spin for the entire scheduler tick.  If that
> > reader belongs to a real-time scheduling class, it can spin forever and
> > the kernel will livelock.
> > 
> > Disabling preemption cannot be done for PREEMPT_RT though: it can lead
> > to higher latencies, and the write side sections will not be able to
> > acquire locks which become sleeping locks (e.g. spinlock_t).
> > 
> > To remain preemptible, while avoiding a possible livelock caused by the
> > reader preempting the writer, use a different technique: let the reader
> > detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
> > case, acquire then release the associated LOCKNAME writer serialization
> > lock. This will allow any possibly-preempted writer to make progress
> > until the end of its writer serialization lock critical section.
> > 
> > Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
> > an associated (PREEMPT_RT) sleeping lock, and for seqlock_t.
> 
> Reverting this patchset [1] from today's linux-next fixed a splat below. The
> splat looks like a false positive anyway because the existing locking dependency
> chains from the task #1 here:
> 
> &s->seqcount#2 ---> pidmap_lock
> 
> [  528.078061][ T7867] -> #1 (pidmap_lock){....}-{2:2}:
> [  528.078078][ T7867]        lock_acquire+0x10c/0x560
> [  528.078089][ T7867]        _raw_spin_lock_irqsave+0x64/0xb0
> [  528.078108][ T7867]        free_pid+0x5c/0x160
> free_pid at kernel/pid.c:131
> [  528.078127][ T7867]        release_task.part.40+0x59c/0x7f0
> __unhash_process at kernel/exit.c:76
> (inlined by) __exit_signal at kernel/exit.c:147
> (inlined by) release_task at kernel/exit.c:198
> [  528.078145][ T7867]        do_exit+0x77c/0xda0
> exit_notify at kernel/exit.c:679
> (inlined by) do_exit at kernel/exit.c:826
> [  528.078163][ T7867]        kthread+0x148/0x1d0
> [  528.078182][ T7867]        ret_from_kernel_thread+0x5c/0x80
> 
> It is write_seqlock(&sig->stats_lock) in __exit_signal(), but the &s->seqcount#2 
> in read_mems_allowed_begin() is read_seqcount_begin(&current->mems_allowed_seq), 
> so there should be no deadlock?
> 

I think this happened because seqcount_##lockname##_init() is defined at
function rather than macro, so when the seqcount_init() gets expand in
that function, the lock_class_key of seqcount will be a static variable
of seqcount_##lockname##_init() function, as a result, all
seqcount_##lockname##_t in the same compile unit (in this case it's
kernel/fork.c) share the same lock class key, and lockdep thought they
are the same lock ;-)

Regards,
Boqun

> [1] git revert --no-edit 0c9794c8b678..1909760f5fc3
> 
> [  528.077900][ T7867] WARNING: possible circular locking dependency detected
> [  528.077912][ T7867] 5.9.0-rc5-next-20200914 #1 Not tainted
> [  528.077921][ T7867] ------------------------------------------------------
> [  528.077931][ T7867] runc:[1:CHILD]/7867 is trying to acquire lock:
> [  528.077942][ T7867] c000001fce5570c8 (&s->seqcount#2){....}-{0:0}, at: __slab_alloc+0x34/0xf0
> [  528.077972][ T7867] 
> [  528.077972][ T7867] but task is already holding lock:
> [  528.077983][ T7867] c0000000056b0198 (pidmap_lock){....}-{2:2}, at: alloc_pid+0x258/0x590
> [  528.078009][ T7867] 
> [  528.078009][ T7867] which lock already depends on the new lock.
> [  528.078009][ T7867] 
> [  528.078031][ T7867] 
> [  528.078031][ T7867] the existing dependency chain (in reverse order) is:
> [  528.078061][ T7867] 
> [  528.078061][ T7867] -> #1 (pidmap_lock){....}-{2:2}:
> [  528.078078][ T7867]        lock_acquire+0x10c/0x560
> [  528.078089][ T7867]        _raw_spin_lock_irqsave+0x64/0xb0
> [  528.078108][ T7867]        free_pid+0x5c/0x160
> free_pid at kernel/pid.c:131
> [  528.078127][ T7867]        release_task.part.40+0x59c/0x7f0
> __unhash_process at kernel/exit.c:76
> (inlined by) __exit_signal at kernel/exit.c:147
> (inlined by) release_task at kernel/exit.c:198
> [  528.078145][ T7867]        do_exit+0x77c/0xda0
> exit_notify at kernel/exit.c:679
> (inlined by) do_exit at kernel/exit.c:826
> [  528.078163][ T7867]        kthread+0x148/0x1d0
> [  528.078182][ T7867]        ret_from_kernel_thread+0x5c/0x80
> [  528.078208][ T7867] 
> [  528.078208][ T7867] -> #0 (&s->seqcount#2){....}-{0:0}:
> [  528.078241][ T7867]        check_prevs_add+0x1c4/0x1120
> check_prev_add at kernel/locking/lockdep.c:2820
> (inlined by) check_prevs_add at kernel/locking/lockdep.c:2944
> [  528.078260][ T7867]        __lock_acquire+0x176c/0x1c00
> validate_chain at kernel/locking/lockdep.c:3562
> (inlined by) __lock_acquire at kernel/locking/lockdep.c:4796
> [  528.078278][ T7867]        lock_acquire+0x10c/0x560
> [  528.078297][ T7867]        ___slab_alloc+0xa40/0xb40
> seqcount_lockdep_reader_access at include/linux/seqlock.h:103
> (inlined by) read_mems_allowed_begin at include/linux/cpuset.h:135
> (inlined by) get_any_partial at mm/slub.c:2035
> (inlined by) get_partial at mm/slub.c:2078
> (inlined by) new_slab_objects at mm/slub.c:2577
> (inlined by) ___slab_alloc at mm/slub.c:2745
> [  528.078324][ T7867]        __slab_alloc+0x34/0xf0
> [  528.078342][ T7867]        kmem_cache_alloc+0x2d4/0x470
> [  528.078362][ T7867]        create_object+0x74/0x430
> [  528.078381][ T7867]        slab_post_alloc_hook+0xa4/0x670
> [  528.078399][ T7867]        kmem_cache_alloc+0x1b4/0x470
> [  528.078418][ T7867]        radix_tree_node_alloc.constprop.19+0xe4/0x160
> [  528.078438][ T7867]        idr_get_free+0x298/0x360
> [  528.078456][ T7867]        idr_alloc_u32+0x84/0x130
> [  528.078474][ T7867]        idr_alloc_cyclic+0x7c/0x150
> [  528.078493][ T7867]        alloc_pid+0x27c/0x590
> [  528.078511][ T7867]        copy_process+0xc90/0x1930
> copy_process at kernel/fork.c:2104
> [  528.078529][ T7867]        kernel_clone+0x120/0xa10
> [  528.078546][ T7867]        __do_sys_clone+0x88/0xd0
> [  528.078565][ T7867]        system_call_exception+0xf8/0x1d0
> [  528.078592][ T7867]        system_call_common+0xe8/0x218
> [  528.078609][ T7867] 
> [  528.078609][ T7867] other info that might help us debug this:
> [  528.078609][ T7867] 
> [  528.078650][ T7867]  Possible unsafe locking scenario:
> [  528.078650][ T7867] 
> [  528.078670][ T7867]        CPU0                    CPU1
> [  528.078695][ T7867]        ----                    ----
> [  528.078713][ T7867]   lock(pidmap_lock);
> [  528.078730][ T7867]                                lock(&s->seqcount#2);
> [  528.078751][ T7867]                                lock(pidmap_lock);
> [  528.078770][ T7867]   lock(&s->seqcount#2);
> [  528.078788][ T7867] 
> [  528.078788][ T7867]  *** DEADLOCK ***
> [  528.078788][ T7867] 
> [  528.078800][ T7867] 2 locks held by runc:[1:CHILD]/7867:
> [  528.078808][ T7867]  #0: c000001ffea6f4f0 (lock#2){+.+.}-{2:2}, at: __radix_tree_preload+0x8/0x370
> __radix_tree_preload at lib/radix-tree.c:322
> [  528.078844][ T7867]  #1: c0000000056b0198 (pidmap_lock){....}-{2:2}, at: alloc_pid+0x258/0x590
> [  528.078870][ T7867] 
> [  528.078870][ T7867] stack backtrace:
> [  528.078890][ T7867] CPU: 46 PID: 7867 Comm: runc:[1:CHILD] Not tainted 5.9.0-rc5-next-20200914 #1
> [  528.078921][ T7867] Call Trace:
> [  528.078940][ T7867] [c000001ff07eefc0] [c00000000063f8c8] dump_stack+0xec/0x144 (unreliable)
> [  528.078964][ T7867] [c000001ff07ef000] [c00000000013f44c] print_circular_bug.isra.43+0x2dc/0x350
> [  528.078978][ T7867] [c000001ff07ef0a0] [c00000000013f640] check_noncircular+0x180/0x1b0
> [  528.079000][ T7867] [c000001ff07ef170] [c000000000140b84] check_prevs_add+0x1c4/0x1120
> [  528.079022][ T7867] [c000001ff07ef280] [c0000000001446ec] __lock_acquire+0x176c/0x1c00
> [  528.079043][ T7867] [c000001ff07ef3a0] [c00000000014578c] lock_acquire+0x10c/0x560
> [  528.079066][ T7867] [c000001ff07ef490] [c0000000003565f0] ___slab_alloc+0xa40/0xb40
> [  528.079079][ T7867] [c000001ff07ef590] [c000000000356724] __slab_alloc+0x34/0xf0
> [  528.079100][ T7867] [c000001ff07ef5e0] [c000000000356ab4] kmem_cache_alloc+0x2d4/0x470
> [  528.079122][ T7867] [c000001ff07ef670] [c000000000397e14] create_object+0x74/0x430
> [  528.079144][ T7867] [c000001ff07ef720] [c000000000351944] slab_post_alloc_hook+0xa4/0x670
> [  528.079165][ T7867] [c000001ff07ef7e0] [c000000000356994] kmem_cache_alloc+0x1b4/0x470
> [  528.079187][ T7867] [c000001ff07ef870] [c00000000064e004] radix_tree_node_alloc.constprop.19+0xe4/0x160
> radix_tree_node_alloc at lib/radix-tree.c:252
> [  528.079219][ T7867] [c000001ff07ef8e0] [c00000000064f2b8] idr_get_free+0x298/0x360
> idr_get_free at lib/radix-tree.c:1507
> [  528.079249][ T7867] [c000001ff07ef970] [c000000000645db4] idr_alloc_u32+0x84/0x130
> idr_alloc_u32 at lib/idr.c:46 (discriminator 4)
> [  528.079271][ T7867] [c000001ff07ef9e0] [c000000000645f8c] idr_alloc_cyclic+0x7c/0x150
> idr_alloc_cyclic at lib/idr.c:126 (discriminator 1)
> [  528.079301][ T7867] [c000001ff07efa40] [c0000000000e48ac] alloc_pid+0x27c/0x590
> [  528.079342][ T7867] [c000001ff07efb20] [c0000000000acc60] copy_process+0xc90/0x1930
> [  528.079404][ T7867] [c000001ff07efc40] [c0000000000adc00] kernel_clone+0x120/0xa10
> [  528.079499][ T7867] [c000001ff07efd00] [c0000000000ae578] __do_sys_clone+0x88/0xd0
> [  528.079579][ T7867] [c000001ff07efdc0] [c000000000029c48] system_call_exception+0xf8/0x1d0
> [  528.079691][ T7867] [c000001ff07efe20] [c00000000000d0a8] system_call_common+0xe8/0x218
> 
> > 
> > 8<--------------
> > 
> > Ahmed S. Darwish (5):
> >   seqlock: seqcount_LOCKNAME_t: Standardize naming convention
> >   seqlock: Use unique prefix for seqcount_t property accessors
> >   seqlock: seqcount_t: Implement all read APIs as statement expressions
> >   seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support
> >   seqlock: PREEMPT_RT: Do not starve seqlock_t writers
> > 
> >  include/linux/seqlock.h | 281 ++++++++++++++++++++++++----------------
> >  1 file changed, 167 insertions(+), 114 deletions(-)
> > 
> > base-commit: f75aef392f869018f78cfedf3c320a6b3fcfda6b
> > --
> > 2.28.0
> 

Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

[Boqun Feng]

On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> On Mon, Sep 14, 2020 at 08:20:53PM -0400, Qian Cai wrote:
> > On Fri, 2020-09-04 at 17:32 +0200, Ahmed S. Darwish wrote:
> > > Hi,
> > > 
> > > Changelog-v2
> > > ============
> > > 
> > >   - Standardize on seqcount_LOCKNAME_t as the canonical reference for
> > >     sequence counters with associated locks, instead of v1
> > >     seqcount_LOCKTYPE_t.
> > > 
> > >   - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t
> > >     property accessors.
> > > 
> > >   - Touch-up the lock-unlock rationale for more clarity. Enforce writer
> > >     non-preemitiblity using "__seq_enforce_writer_non_preemptibility()".
> > > 
> > > Cover letter (v1)
> > > =================
> > > 
> > > https://lkml.kernel.org/r/20200828010710.5407-1-a.darwish@linutronix.de
> > > 
> > > Preemption must be disabled before entering a sequence counter write
> > > side critical section.  Otherwise the read side section can preempt the
> > > write side section and spin for the entire scheduler tick.  If that
> > > reader belongs to a real-time scheduling class, it can spin forever and
> > > the kernel will livelock.
> > > 
> > > Disabling preemption cannot be done for PREEMPT_RT though: it can lead
> > > to higher latencies, and the write side sections will not be able to
> > > acquire locks which become sleeping locks (e.g. spinlock_t).
> > > 
> > > To remain preemptible, while avoiding a possible livelock caused by the
> > > reader preempting the writer, use a different technique: let the reader
> > > detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
> > > case, acquire then release the associated LOCKNAME writer serialization
> > > lock. This will allow any possibly-preempted writer to make progress
> > > until the end of its writer serialization lock critical section.
> > > 
> > > Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
> > > an associated (PREEMPT_RT) sleeping lock, and for seqlock_t.
> > 
> > Reverting this patchset [1] from today's linux-next fixed a splat below. The
> > splat looks like a false positive anyway because the existing locking dependency
> > chains from the task #1 here:
> > 
> > &s->seqcount#2 ---> pidmap_lock
> > 
> > [  528.078061][ T7867] -> #1 (pidmap_lock){....}-{2:2}:
> > [  528.078078][ T7867]        lock_acquire+0x10c/0x560
> > [  528.078089][ T7867]        _raw_spin_lock_irqsave+0x64/0xb0
> > [  528.078108][ T7867]        free_pid+0x5c/0x160
> > free_pid at kernel/pid.c:131
> > [  528.078127][ T7867]        release_task.part.40+0x59c/0x7f0
> > __unhash_process at kernel/exit.c:76
> > (inlined by) __exit_signal at kernel/exit.c:147
> > (inlined by) release_task at kernel/exit.c:198
> > [  528.078145][ T7867]        do_exit+0x77c/0xda0
> > exit_notify at kernel/exit.c:679
> > (inlined by) do_exit at kernel/exit.c:826
> > [  528.078163][ T7867]        kthread+0x148/0x1d0
> > [  528.078182][ T7867]        ret_from_kernel_thread+0x5c/0x80
> > 
> > It is write_seqlock(&sig->stats_lock) in __exit_signal(), but the &s->seqcount#2 
> > in read_mems_allowed_begin() is read_seqcount_begin(&current->mems_allowed_seq), 
> > so there should be no deadlock?
> > 
> 
> I think this happened because seqcount_##lockname##_init() is defined at
> function rather than macro, so when the seqcount_init() gets expand in
> that function, the lock_class_key of seqcount will be a static variable
> of seqcount_##lockname##_init() function, as a result, all
> seqcount_##lockname##_t in the same compile unit (in this case it's
> kernel/fork.c) share the same lock class key, and lockdep thought they
> are the same lock ;-)
> 

Don't know how to fix this properly, but below is an ugly attemption,
only build test, just food for thoughts.

Regards,
Boqun

--------------->8
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index f73c7eb68f27..938a5053def3 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -84,14 +84,18 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
 # define SEQCOUNT_DEP_MAP_INIT(lockname)				\
 		.dep_map = { .name = #lockname }
 
+# define MSIOCU 8 /* MAX SEQCOUNT IN ON COMPILE UNIT */
 /**
  * seqcount_init() - runtime initializer for seqcount_t
  * @s: Pointer to the seqcount_t instance
  */
 # define seqcount_init(s)						\
 	do {								\
-		static struct lock_class_key __key;			\
-		__seqcount_init((s), #s, &__key);			\
+		static struct lock_class_key __key[MSIOCU];		\
+		static int idx = 0;					\
+									\
+		BUG_ON(idx >= MSIOCU);					\
+		__seqcount_init((s), #s, &__key[idx++]);		\
 	} while (0)
 
 static inline void seqcount_lockdep_reader_access(const seqcount_t *s)

Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

[peterz]

On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> I think this happened because seqcount_##lockname##_init() is defined at
> function rather than macro, so when the seqcount_init() gets expand in

Bah! I hate all this :/

I suspect the below, while more verbose than I'd like is the best
option.

---
 include/linux/seqlock.h | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index f73c7eb68f27..76e44e6c0100 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -173,6 +173,19 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
  * @lock:	Pointer to the associated lock
  */
 
+#define seqcount_LOCKNAME_init(s, _lock, lockname)			\
+	do {								\
+		seqcount_##lockname##_t *____s = (s);			\
+		seqcount_init(&____s->seqcount);			\
+		__SEQ_LOCK(____s->lock = (_lock));			\
+	} while (0)
+
+#define seqcount_raw_spinlock_init(s, lock)	seqcount_LOCKNAME_init(s, lock, raw_spinlock)
+#define seqcount_spinlock_init(s, lock)		seqcount_LOCKNAME_init(s, lock, spinlock)
+#define seqcount_rwlock_init(s, lock)		seqcount_LOCKNAME_init(s, lock, rwlock);
+#define seqcount_mutex_init(s, lock)		seqcount_LOCKNAME_init(s, lock, mutex);
+#define seqcount_ww_mutex_init(s, lock)		seqcount_LOCKNAME_init(s, lock, ww_mutex);
+
 /*
  * SEQCOUNT_LOCKNAME()	- Instantiate seqcount_LOCKNAME_t and helpers
  * seqprop_LOCKNAME_*()	- Property accessors for seqcount_LOCKNAME_t
@@ -190,13 +203,6 @@ typedef struct seqcount_##lockname {					\
 	__SEQ_LOCK(locktype	*lock);					\
 } seqcount_##lockname##_t;						\
 									\
-static __always_inline void						\
-seqcount_##lockname##_init(seqcount_##lockname##_t *s, locktype *lock)	\
-{									\
-	seqcount_init(&s->seqcount);					\
-	__SEQ_LOCK(s->lock = lock);					\
-}									\
-									\
 static __always_inline seqcount_t *					\
 __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s)			\
 {									\
@@ -284,8 +290,8 @@ SEQCOUNT_LOCKNAME(ww_mutex,     struct ww_mutex, true,     &s->lock->base, ww_mu
 	__SEQ_LOCK(.lock	= (assoc_lock))				\
 }
 
-#define SEQCNT_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name, lock)
 #define SEQCNT_RAW_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name, lock)
+#define SEQCNT_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name, lock)
 #define SEQCNT_RWLOCK_ZERO(name, lock)		SEQCOUNT_LOCKNAME_ZERO(name, lock)
 #define SEQCNT_MUTEX_ZERO(name, lock)		SEQCOUNT_LOCKNAME_ZERO(name, lock)
 #define SEQCNT_WW_MUTEX_ZERO(name, lock) 	SEQCOUNT_LOCKNAME_ZERO(name, lock)

Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

[Qian Cai]

On Tue, 2020-09-15 at 16:30 +0200, peterz@infradead.org wrote:
> On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> > I think this happened because seqcount_##lockname##_init() is defined at
> > function rather than macro, so when the seqcount_init() gets expand in
> 
> Bah! I hate all this :/
> 
> I suspect the below, while more verbose than I'd like is the best
> option.

Stephen, can you add this patch for now until Peter beats you to it?

> 
> ---
>  include/linux/seqlock.h | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index f73c7eb68f27..76e44e6c0100 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -173,6 +173,19 @@ static inline void seqcount_lockdep_reader_access(const
> seqcount_t *s)
>   * @lock:	Pointer to the associated lock
>   */
>  
> +#define seqcount_LOCKNAME_init(s, _lock, lockname)			\
> +	do {								\
> +		seqcount_##lockname##_t *____s = (s);			\
> +		seqcount_init(&____s->seqcount);			\
> +		__SEQ_LOCK(____s->lock = (_lock));			\
> +	} while (0)
> +
> +#define seqcount_raw_spinlock_init(s, lock)	seqcount_LOCKNAME_init(s, lock,
> raw_spinlock)
> +#define seqcount_spinlock_init(s, lock)		seqcount_LOCKNAME_init(s
> , lock, spinlock)
> +#define seqcount_rwlock_init(s, lock)		seqcount_LOCKNAME_init(s
> , lock, rwlock);
> +#define seqcount_mutex_init(s, lock)		seqcount_LOCKNAME_init(s, lock,
> mutex);
> +#define seqcount_ww_mutex_init(s, lock)		seqcount_LOCKNAME_init(s
> , lock, ww_mutex);
> +
>  /*
>   * SEQCOUNT_LOCKNAME()	- Instantiate seqcount_LOCKNAME_t and helpers
>   * seqprop_LOCKNAME_*()	- Property accessors for seqcount_LOCKNAME_t
> @@ -190,13 +203,6 @@ typedef struct seqcount_##lockname {				
> 	\
>  	__SEQ_LOCK(locktype	*lock);					\
>  } seqcount_##lockname##_t;						\
>  									\
> -static __always_inline void						\
> -seqcount_##lockname##_init(seqcount_##lockname##_t *s, locktype *lock)	
> \
> -{									\
> -	seqcount_init(&s->seqcount);					\
> -	__SEQ_LOCK(s->lock = lock);					\
> -}									\
> -									\
>  static __always_inline seqcount_t *					\
>  __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s)			
> \
>  {									\
> @@ -284,8 +290,8 @@ SEQCOUNT_LOCKNAME(ww_mutex,     struct ww_mutex,
> true,     &s->lock->base, ww_mu
>  	__SEQ_LOCK(.lock	= (assoc_lock))				\
>  }
>  
> -#define SEQCNT_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name,
> lock)
>  #define SEQCNT_RAW_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name,
> lock)
> +#define SEQCNT_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name,
> lock)
>  #define SEQCNT_RWLOCK_ZERO(name, lock)		SEQCOUNT_LOCKNAME_ZERO(n
> ame, lock)
>  #define SEQCNT_MUTEX_ZERO(name, lock)		SEQCOUNT_LOCKNAME_ZERO(n
> ame, lock)
>  #define SEQCNT_WW_MUTEX_ZERO(name, lock) 	SEQCOUNT_LOCKNAME_ZERO(name,
> lock)
> 

Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

[peterz]

On Wed, Sep 16, 2020 at 08:52:07AM -0400, Qian Cai wrote:
> On Tue, 2020-09-15 at 16:30 +0200, peterz@infradead.org wrote:
> > On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> > > I think this happened because seqcount_##lockname##_init() is defined at
> > > function rather than macro, so when the seqcount_init() gets expand in
> > 
> > Bah! I hate all this :/
> > 
> > I suspect the below, while more verbose than I'd like is the best
> > option.
> 
> Stephen, can you add this patch for now until Peter beats you to it?

Did you verify it works? I only wrote it..

Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

[Qian Cai]

----- Original Message -----
> On Wed, Sep 16, 2020 at 08:52:07AM -0400, Qian Cai wrote:
> > On Tue, 2020-09-15 at 16:30 +0200, peterz@infradead.org wrote:
> > > On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> > > > I think this happened because seqcount_##lockname##_init() is defined
> > > > at
> > > > function rather than macro, so when the seqcount_init() gets expand in
> > > 
> > > Bah! I hate all this :/
> > > 
> > > I suspect the below, while more verbose than I'd like is the best
> > > option.
> > 
> > Stephen, can you add this patch for now until Peter beats you to it?
> 
> Did you verify it works? I only wrote it..

Yes, I did.

Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

[peterz]

On Wed, Sep 16, 2020 at 09:00:59AM -0400, Qian Cai wrote:
> 
> 
> ----- Original Message -----
> > On Wed, Sep 16, 2020 at 08:52:07AM -0400, Qian Cai wrote:
> > > On Tue, 2020-09-15 at 16:30 +0200, peterz@infradead.org wrote:
> > > > On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> > > > > I think this happened because seqcount_##lockname##_init() is defined
> > > > > at
> > > > > function rather than macro, so when the seqcount_init() gets expand in
> > > > 
> > > > Bah! I hate all this :/
> > > > 
> > > > I suspect the below, while more verbose than I'd like is the best
> > > > option.
> > > 
> > > Stephen, can you add this patch for now until Peter beats you to it?
> > 
> > Did you verify it works? I only wrote it..
> 
> Yes, I did.

Excellent, I'll stick a Tested-by from you on then.

Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

[Stephen Rothwell]

[-- Attachment #1: Type: text/plain, Size: 1110 bytes --]

Hi all,

On Wed, 16 Sep 2020 15:02:33 +0200 peterz@infradead.org wrote:
>
> On Wed, Sep 16, 2020 at 09:00:59AM -0400, Qian Cai wrote:
> > 
> > 
> > ----- Original Message -----  
> > > On Wed, Sep 16, 2020 at 08:52:07AM -0400, Qian Cai wrote:  
> > > > On Tue, 2020-09-15 at 16:30 +0200, peterz@infradead.org wrote:  
> > > > > On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:  
> > > > > > I think this happened because seqcount_##lockname##_init() is defined
> > > > > > at
> > > > > > function rather than macro, so when the seqcount_init() gets expand in  
> > > > > 
> > > > > Bah! I hate all this :/
> > > > > 
> > > > > I suspect the below, while more verbose than I'd like is the best
> > > > > option.  
> > > > 
> > > > Stephen, can you add this patch for now until Peter beats you to it?  
> > > 
> > > Did you verify it works? I only wrote it..  
> > 
> > Yes, I did.  
> 
> Excellent, I'll stick a Tested-by from you on then.

I'll add this into the tip tree merge today (unless the tip tree is
updated in the mean time).

-- 
Cheers,
Stephen Rothwell

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[tip: locking/core] seqlock: Unbreak lockdep

[tip-bot2 for peterz@infradead.org]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     267580db047ef428a70bef8287ca62c5a450c139
Gitweb:        https://git.kernel.org/tip/267580db047ef428a70bef8287ca62c5a450c139
Author:        peterz@infradead.org <peterz@infradead.org>
AuthorDate:    Tue, 15 Sep 2020 16:30:28 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 16 Sep 2020 16:26:58 +02:00

seqlock: Unbreak lockdep

seqcount_LOCKNAME_init() needs to be a macro due to the lockdep
annotation in seqcount_init(). Since a macro cannot define another
macro, we need to effectively revert commit: e4e9ab3f9f91 ("seqlock:
Fold seqcount_LOCKNAME_init() definition").

Fixes: e4e9ab3f9f91 ("seqlock: Fold seqcount_LOCKNAME_init() definition")
Reported-by: Qian Cai <cai@redhat.com>
Debugged-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Qian Cai <cai@redhat.com>
Link: https://lkml.kernel.org/r/20200915143028.GB2674@hirez.programming.kicks-ass.net
---
 include/linux/seqlock.h | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index f73c7eb..76e44e6 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -173,6 +173,19 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
  * @lock:	Pointer to the associated lock
  */
 
+#define seqcount_LOCKNAME_init(s, _lock, lockname)			\
+	do {								\
+		seqcount_##lockname##_t *____s = (s);			\
+		seqcount_init(&____s->seqcount);			\
+		__SEQ_LOCK(____s->lock = (_lock));			\
+	} while (0)
+
+#define seqcount_raw_spinlock_init(s, lock)	seqcount_LOCKNAME_init(s, lock, raw_spinlock)
+#define seqcount_spinlock_init(s, lock)		seqcount_LOCKNAME_init(s, lock, spinlock)
+#define seqcount_rwlock_init(s, lock)		seqcount_LOCKNAME_init(s, lock, rwlock);
+#define seqcount_mutex_init(s, lock)		seqcount_LOCKNAME_init(s, lock, mutex);
+#define seqcount_ww_mutex_init(s, lock)		seqcount_LOCKNAME_init(s, lock, ww_mutex);
+
 /*
  * SEQCOUNT_LOCKNAME()	- Instantiate seqcount_LOCKNAME_t and helpers
  * seqprop_LOCKNAME_*()	- Property accessors for seqcount_LOCKNAME_t
@@ -190,13 +203,6 @@ typedef struct seqcount_##lockname {					\
 	__SEQ_LOCK(locktype	*lock);					\
 } seqcount_##lockname##_t;						\
 									\
-static __always_inline void						\
-seqcount_##lockname##_init(seqcount_##lockname##_t *s, locktype *lock)	\
-{									\
-	seqcount_init(&s->seqcount);					\
-	__SEQ_LOCK(s->lock = lock);					\
-}									\
-									\
 static __always_inline seqcount_t *					\
 __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s)			\
 {									\
@@ -284,8 +290,8 @@ SEQCOUNT_LOCKNAME(ww_mutex,     struct ww_mutex, true,     &s->lock->base, ww_mu
 	__SEQ_LOCK(.lock	= (assoc_lock))				\
 }
 
-#define SEQCNT_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name, lock)
 #define SEQCNT_RAW_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name, lock)
+#define SEQCNT_SPINLOCK_ZERO(name, lock)	SEQCOUNT_LOCKNAME_ZERO(name, lock)
 #define SEQCNT_RWLOCK_ZERO(name, lock)		SEQCOUNT_LOCKNAME_ZERO(name, lock)
 #define SEQCNT_MUTEX_ZERO(name, lock)		SEQCOUNT_LOCKNAME_ZERO(name, lock)
 #define SEQCNT_WW_MUTEX_ZERO(name, lock) 	SEQCOUNT_LOCKNAME_ZERO(name, lock)