Commit 55eed755 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar

locking/seqcount: Re-fix raw_read_seqcount_latch()

Commit 50755bc1 ("seqlock: fix raw_read_seqcount_latch()") broke
raw_read_seqcount_latch().

If you look at the comment that was modified; the thing that changes is
the seq count, not the latch pointer.

 * void latch_modify(struct latch_struct *latch, ...)
 * {
 *	smp_wmb();	<- Ensure that the last data[1] update is visible
 *	latch->seq++;
 *	smp_wmb();	<- Ensure that the seqcount update is visible
 *
 *	modify(latch->data[0], ...);
 *
 *	smp_wmb();	<- Ensure that the data[0] update is visible
 *	latch->seq++;
 *	smp_wmb();	<- Ensure that the seqcount update is visible
 *
 *	modify(latch->data[1], ...);
 * }
 *
 * The query will have a form like:
 *
 * struct entry *latch_query(struct latch_struct *latch, ...)
 * {
 *	struct entry *entry;
 *	unsigned seq, idx;
 *
 *	do {
 *		seq = lockless_dereference(latch->seq);

So here we have:

		seq = READ_ONCE(latch->seq);
		smp_read_barrier_depends();

Which is exactly what we want; the new code:

		seq = ({ p = READ_ONCE(latch);
			 smp_read_barrier_depends(); p })->seq;

is just wrong; because it looses the volatile read on seq, which can now
be torn or worse 'optimized'. And the read_depend barrier is also placed
wrong, we want it after the load of seq, to match the above data[]
up-to-date wmb()s.

Such that when we dereference latch->data[] below, we're guaranteed to
observe the right data.

 *
 *		idx = seq & 0x01;
 *		entry = data_query(latch->data[idx], ...);
 *
 *		smp_rmb();
 *	} while (seq != latch->seq);
 *
 *	return entry;
 * }

So yes, not passing a pointer is not pretty, but the code was correct,
and isn't anymore now.

Change to explicit READ_ONCE()+smp_read_barrier_depends() to avoid
confusion and allow strict lockless_dereference() checking.
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 50755bc1 ("seqlock: fix raw_read_seqcount_latch()")
Link: http://lkml.kernel.org/r/20160527111117.GL3192@twins.programming.kicks-ass.netSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 719af93a
...@@ -277,7 +277,10 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s) ...@@ -277,7 +277,10 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
static inline int raw_read_seqcount_latch(seqcount_t *s) static inline int raw_read_seqcount_latch(seqcount_t *s)
{ {
return lockless_dereference(s)->sequence; int seq = READ_ONCE(s->sequence);
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
smp_read_barrier_depends();
return seq;
} }
/** /**
...@@ -331,7 +334,7 @@ static inline int raw_read_seqcount_latch(seqcount_t *s) ...@@ -331,7 +334,7 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* unsigned seq, idx; * unsigned seq, idx;
* *
* do { * do {
* seq = lockless_dereference(latch)->seq; * seq = raw_read_seqcount_latch(&latch->seq);
* *
* idx = seq & 0x01; * idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...); * entry = data_query(latch->data[idx], ...);
......
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