Re: [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 04, 2020 at 04:17:11AM +0100, Ahmed S. Darwish wrote:
> On Tue, Nov 03, 2020 at 06:01:30PM -0800, John Hubbard wrote:
> > On 11/3/20 5:32 PM, Ahmed S. Darwish wrote:
> ...
> > >   #define __read_seqcount_retry(s, start)					\
> > > -	__read_seqcount_t_retry(__seqcount_ptr(s), start)
> > > +	__do___read_seqcount_retry(__seqcount_ptr(s), start)
> >
> ...
> > A nit: while various numbers of leading underscores are sometimes used, it's a lot
> > less common to use, say, 3 consecutive underscores (as above) *within* the name. And
> > I don't think you need it for uniqueness, at least from a quick look around here.
> >
> ...
> > But again, either way, I think "do" is helping a *lot* here (as is getting rid
> > of the _t_ idea).
> 
> The three underscores are needed because there's a do_ version for
> read_seqcount_retry(), and another for __read_seqcount_retry().
> 
> Similarly for {__,}read_seqcount_begin(). You want to be very careful
> with this, and never mistaknely mix the two, because it affects some VFS
> hot paths.
> 
> Nonetheless, as you mentioned in the later (dropped) part of your
> message, I think do_ is better than __do_, so the final result will be:
> 
>   do___read_seqcount_retry()
>   do_read_seqcount_retry()
>   do_raw_write_seqcount_begin()
>   do_raw_write_seqcount_end()
>   do_write_seqcount_begin()
>   ...
> 
> and so on.
> 
> I'll wait for some further feedback on the two patches (possibly from
> Linus or PeterZ), then send a mini patch series.

I'm Ok with that. The change I have issues with is:

-#define __seqcount_lock_preemptible(s) __seqprop(s, preemptible)
+#define __seqcount_associated_lock_exists_and_is_preemptible(s)        __seqprop(s, preemptible)

That's just _realllllllly_ long.

Would something like the below make it easier to follow?

seqprop_preemptible() is 'obviously' related to __seqprop_preemptible()
without having to follow through the _Generic token pasting maze.

---
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 8d8552474c64..576594add921 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -307,10 +307,10 @@ SEQCOUNT_LOCKNAME(ww_mutex,     struct ww_mutex, true,     &s->lock->base, ww_mu
 	__seqprop_case((s),	mutex,		prop),			\
 	__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)
+#define seqprop_ptr(s)			__seqprop(s, ptr)
+#define seqprop_sequence(s)		__seqprop(s, sequence)
+#define seqprop_preemptible(s)		__seqprop(s, preemptible)
+#define seqprop_assert(s)	__seqprop(s, assert)
 
 /**
  * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
@@ -330,7 +330,7 @@ SEQCOUNT_LOCKNAME(ww_mutex,     struct ww_mutex, true,     &s->lock->base, ww_mu
 ({									\
 	unsigned __seq;							\
 									\
-	while ((__seq = __seqcount_sequence(s)) & 1)			\
+	while ((__seq = seqprop_sequence(s)) & 1)			\
 		cpu_relax();						\
 									\
 	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);			\
@@ -359,7 +359,7 @@ SEQCOUNT_LOCKNAME(ww_mutex,     struct ww_mutex, true,     &s->lock->base, ww_mu
  */
 #define read_seqcount_begin(s)						\
 ({									\
-	seqcount_lockdep_reader_access(__seqcount_ptr(s));		\
+	seqcount_lockdep_reader_access(seqprop_ptr(s));			\
 	raw_read_seqcount_begin(s);					\
 })
 
@@ -376,7 +376,7 @@ SEQCOUNT_LOCKNAME(ww_mutex,     struct ww_mutex, true,     &s->lock->base, ww_mu
  */
 #define raw_read_seqcount(s)						\
 ({									\
-	unsigned __seq = __seqcount_sequence(s);			\
+	unsigned __seq = seqprop_sequence(s);				\
 									\
 	smp_rmb();							\
 	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);			\
@@ -425,7 +425,7 @@ SEQCOUNT_LOCKNAME(ww_mutex,     struct ww_mutex, true,     &s->lock->base, ww_mu
  * Return: true if a read section retry is required, else false
  */
 #define __read_seqcount_retry(s, start)					\
-	__read_seqcount_t_retry(__seqcount_ptr(s), start)
+	__read_seqcount_t_retry(seqprop_ptr(s), start)
 
 static inline int __read_seqcount_t_retry(const seqcount_t *s, unsigned start)
 {
@@ -445,7 +445,7 @@ static inline int __read_seqcount_t_retry(const seqcount_t *s, unsigned start)
  * Return: true if a read section retry is required, else false
  */
 #define read_seqcount_retry(s, start)					\
-	read_seqcount_t_retry(__seqcount_ptr(s), start)
+	read_seqcount_t_retry(seqprop_ptr(s), start)
 
 static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
 {
@@ -459,10 +459,10 @@ static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
  */
 #define raw_write_seqcount_begin(s)					\
 do {									\
-	if (__seqcount_lock_preemptible(s))				\
+	if (seqprop_preemptible(s))					\
 		preempt_disable();					\
 									\
-	raw_write_seqcount_t_begin(__seqcount_ptr(s));			\
+	raw_write_seqcount_t_begin(seqprop_ptr(s));			\
 } while (0)
 
 static inline void raw_write_seqcount_t_begin(seqcount_t *s)
@@ -478,9 +478,9 @@ static inline void raw_write_seqcount_t_begin(seqcount_t *s)
  */
 #define raw_write_seqcount_end(s)					\
 do {									\
-	raw_write_seqcount_t_end(__seqcount_ptr(s));			\
+	raw_write_seqcount_t_end(seqprop_ptr(s));			\
 									\
-	if (__seqcount_lock_preemptible(s))				\
+	if (seqprop_preemptible(s))					\
 		preempt_enable();					\
 } while (0)
 
@@ -501,12 +501,12 @@ static inline void raw_write_seqcount_t_end(seqcount_t *s)
  */
 #define write_seqcount_begin_nested(s, subclass)			\
 do {									\
-	__seqcount_assert_lock_held(s);					\
+	seqprop_assert(s);						\
 									\
-	if (__seqcount_lock_preemptible(s))				\
+	if (seqprop_preemptible(s))					\
 		preempt_disable();					\
 									\
-	write_seqcount_t_begin_nested(__seqcount_ptr(s), subclass);	\
+	write_seqcount_t_begin_nested(seqprop_ptr(s), subclass);	\
 } while (0)
 
 static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
@@ -528,12 +528,12 @@ static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
  */
 #define write_seqcount_begin(s)						\
 do {									\
-	__seqcount_assert_lock_held(s);					\
+	seqprop_assert(s);						\
 									\
-	if (__seqcount_lock_preemptible(s))				\
+	if (seqprop_preemptible(s))					\
 		preempt_disable();					\
 									\
-	write_seqcount_t_begin(__seqcount_ptr(s));			\
+	write_seqcount_t_begin(seqprop_ptr(s));				\
 } while (0)
 
 static inline void write_seqcount_t_begin(seqcount_t *s)
@@ -549,9 +549,9 @@ static inline void write_seqcount_t_begin(seqcount_t *s)
  */
 #define write_seqcount_end(s)						\
 do {									\
-	write_seqcount_t_end(__seqcount_ptr(s));			\
+	write_seqcount_t_end(seqprop_ptr(s));				\
 									\
-	if (__seqcount_lock_preemptible(s))				\
+	if (seqprop_preemptible(s))					\
 		preempt_enable();					\
 } while (0)
 
@@ -603,7 +603,7 @@ static inline void write_seqcount_t_end(seqcount_t *s)
  *      }
  */
 #define raw_write_seqcount_barrier(s)					\
-	raw_write_seqcount_t_barrier(__seqcount_ptr(s))
+	raw_write_seqcount_t_barrier(seqprop_ptr(s))
 
 static inline void raw_write_seqcount_t_barrier(seqcount_t *s)
 {
@@ -623,7 +623,7 @@ static inline void raw_write_seqcount_t_barrier(seqcount_t *s)
  * will complete successfully and see data older than this.
  */
 #define write_seqcount_invalidate(s)					\
-	write_seqcount_t_invalidate(__seqcount_ptr(s))
+	write_seqcount_t_invalidate(seqprop_ptr(s))
 
 static inline void write_seqcount_t_invalidate(seqcount_t *s)
 {