From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
"Sebastian A. Siewior" <bigeasy@linutronix.de>,
"Paul E. McKenney" <paulmck@kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
LKML <linux-kernel@vger.kernel.org>,
"Ahmed S. Darwish" <a.darwish@linutronix.de>
Subject: [PATCH v2 5/5] seqlock: PREEMPT_RT: Do not starve seqlock_t writers
Date: Fri, 4 Sep 2020 17:32:31 +0200 [thread overview]
Message-ID: <20200904153231.11994-6-a.darwish@linutronix.de> (raw)
In-Reply-To: <20200904153231.11994-1-a.darwish@linutronix.de>
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
next prev parent reply other threads:[~2020-09-04 15:33 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-04 15:32 [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support Ahmed S. Darwish
2020-09-04 15:32 ` [PATCH v2 1/5] seqlock: seqcount_LOCKNAME_t: Standardize naming convention Ahmed S. Darwish
2020-09-10 15:08 ` [tip: locking/core] " tip-bot2 for Ahmed S. Darwish
2020-09-04 15:32 ` [PATCH v2 2/5] seqlock: Use unique prefix for seqcount_t property accessors Ahmed S. Darwish
2020-09-08 11:41 ` peterz
2020-09-10 15:08 ` [tip: locking/core] " tip-bot2 for Ahmed S. Darwish
2020-09-04 15:32 ` [PATCH v2 3/5] seqlock: seqcount_t: Implement all read APIs as statement expressions Ahmed S. Darwish
2020-09-10 15:08 ` [tip: locking/core] " tip-bot2 for Ahmed S. Darwish
2020-09-04 15:32 ` [PATCH v2 4/5] seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support Ahmed S. Darwish
2020-09-08 11:45 ` peterz
2020-09-08 12:48 ` Ahmed S. Darwish
2020-09-04 15:32 ` Ahmed S. Darwish [this message]
2020-09-10 15:08 ` [tip: locking/core] seqlock: PREEMPT_RT: Do not starve seqlock_t writers tip-bot2 for Ahmed S. Darwish
2020-09-15 0:20 ` [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support Qian Cai
2020-09-15 12:48 ` Boqun Feng
2020-09-15 13:10 ` Boqun Feng
2020-09-15 14:30 ` peterz
2020-09-16 12:52 ` Qian Cai
2020-09-16 12:54 ` peterz
2020-09-16 13:00 ` Qian Cai
2020-09-16 13:02 ` peterz
2020-09-17 2:31 ` Stephen Rothwell
2020-09-18 8:42 ` [tip: locking/core] seqlock: Unbreak lockdep tip-bot2 for peterz@infradead.org
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