Messages in this thread Patch in this message | | | From | Manfred Spraul <> | Subject | [PATCH 1/3] ipc/sem.c: Chance memory barrier in sem_lock() to smp_rmb() | Date | Mon, 6 Oct 2014 20:32:41 +0200 |
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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.
Signed-off-by: Manfred Spraul <manfred@colorfullife.com> --- ipc/sem.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/ipc/sem.c b/ipc/sem.c index 454f6c6..ffc71de 100644 --- a/ipc/sem.c +++ b/ipc/sem.c @@ -326,10 +326,16 @@ 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 next test must happen after the test for + * sem_perm.lock, 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. */ -- 1.9.3
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