From: tip-bot for Peter Zijlstra <tipbot@zytor.com>
To: linux-tip-commits@vger.kernel.org
Cc: peterz@infradead.org, adobriyan@gmail.com, hpa@zytor.com,
paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org,
torvalds@linux-foundation.org, tglx@linutronix.de,
akpm@linux-foundation.org, mingo@kernel.org
Subject: [tip:locking/core] locking/seqcount: Re-fix raw_read_seqcount_latch()
Date: Fri, 3 Jun 2016 03:46:15 -0700 [thread overview]
Message-ID: <tip-55eed755c6e30a89be3a791a6b0ad208aadd9bdc@git.kernel.org> (raw)
In-Reply-To: <20160527111117.GL3192@twins.programming.kicks-ass.net>
Commit-ID: 55eed755c6e30a89be3a791a6b0ad208aadd9bdc
Gitweb: http://git.kernel.org/tip/55eed755c6e30a89be3a791a6b0ad208aadd9bdc
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Fri, 27 May 2016 13:11:17 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 3 Jun 2016 08:37:25 +0200
locking/seqcount: Re-fix raw_read_seqcount_latch()
Commit 50755bc1c305 ("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: Peter 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: 50755bc1c305 ("seqlock: fix raw_read_seqcount_latch()")
Link: http://lkml.kernel.org/r/20160527111117.GL3192@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
include/linux/seqlock.h | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 7973a82..ead9765 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -277,7 +277,10 @@ static inline void raw_write_seqcount_barrier(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)
* unsigned seq, idx;
*
* do {
- * seq = lockless_dereference(latch)->seq;
+ * seq = raw_read_seqcount_latch(&latch->seq);
*
* idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...);
prev parent reply other threads:[~2016-06-03 10:47 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-21 20:14 [PATCH] seqlock: fix raw_read_seqcount_latch() Alexey Dobriyan
2016-05-22 10:48 ` Peter Zijlstra
2016-05-22 18:50 ` Alexey Dobriyan
2016-05-23 9:36 ` Peter Zijlstra
2016-05-25 19:57 ` Tejun Heo
2016-05-25 20:11 ` [PATCH] percpu: Revert ("percpu: Replace smp_read_barrier_depends() with lockless_dereference()") Tejun Heo
2016-06-03 10:58 ` [tip:locking/core] locking/barriers: Validate lockless_dereference() is used on a pointer type tip-bot for Peter Zijlstra
2016-06-06 21:31 ` Mateusz Guzik
2016-06-07 7:10 ` Peter Zijlstra
2016-06-08 14:19 ` tip-bot for Peter Zijlstra
2016-05-27 11:11 ` [PATCH] seqlock: fix raw_read_seqcount_latch() Peter Zijlstra
2016-06-03 10:46 ` tip-bot for Peter Zijlstra [this message]
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