| From | Luis Henriques <> | Subject | [PATCH 3.16.y-ckt 112/130] ipc/sem.c: change memory barrier in sem_lock() to smp_rmb() | Date | Fri, 4 Sep 2015 14:08:20 +0100 |
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3.16.7-ckt17 -stable review patch. If anyone has any objections, please let me know.
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From: Manfred Spraul <manfred@colorfullife.com>
commit 2e094abfd1f29a08a60523b42d4508281b8dee0e upstream.
When I fixed bugs in the sem_lock() logic, I was more conservative than necessary. Therefore it is safe to replace the smp_mb() with smp_rmb(). And: With smp_rmb(), semop() syscalls are up to 10% faster.
The race we must protect against is:
sem->lock is free sma->complex_count = 0 sma->sem_perm.lock held by thread B
thread A:
A: spin_lock(&sem->lock)
B: sma->complex_count++; (now 1) B: spin_unlock(&sma->sem_perm.lock);
A: spin_is_locked(&sma->sem_perm.lock); A: XXXXX memory barrier A: if (sma->complex_count == 0)
Thread A must read the increased complex_count value, i.e. the read must not be reordered with the read of sem_perm.lock done by spin_is_locked().
Since it's about ordering of reads, smp_rmb() is sufficient.
[akpm@linux-foundation.org: update sem_lock() comment, from Davidlohr] Signed-off-by: Manfred Spraul <manfred@colorfullife.com> Reviewed-by: Davidlohr Bueso <dave@stgolabs.net> Acked-by: Rafael Aquini <aquini@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> [ luis: 3.16 prereq for: 3ed1f8a99d70 "ipc/sem.c: update/correct memory barriers" ] Signed-off-by: Luis Henriques <luis.henriques@canonical.com> --- ipc/sem.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-)
diff --git a/ipc/sem.c b/ipc/sem.c index 068897b50ffa..f64efdd19586 100644 --- a/ipc/sem.c +++ b/ipc/sem.c @@ -326,10 +326,17 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops, /* Then check that the global lock is free */ if (!spin_is_locked(&sma->sem_perm.lock)) { - /* spin_is_locked() is not a memory barrier */ - smp_mb(); + /* + * The ipc object lock check must be visible on all + * cores before rechecking the complex count. Otherwise + * we can race with another thread that does: + * complex_count++; + * spin_unlock(sem_perm.lock); + */ + smp_rmb(); - /* Now repeat the test of complex_count: + /* + * Now repeat the test of complex_count: * It can't change anymore until we drop sem->lock. * Thus: if is now 0, then it will stay 0. */
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