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| author | Manfred Spraul <manfred@colorfullife.com> | 2014-12-12 16:58:11 -0800 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2014-12-13 12:42:52 -0800 |
| commit | 2e094abfd1f29a08a60523b42d4508281b8dee0e (patch) | |
| tree | 60c10635e14ebc3065b1a40e62517244d929409b /ipc | |
| parent | a060bfe032bcb8522b470f8a7a16e225a9fe5dd6 (diff) | |
| download | linux-riscv-2e094abfd1f29a08a60523b42d4508281b8dee0e.tar.gz linux-riscv-2e094abfd1f29a08a60523b42d4508281b8dee0e.tar.bz2 linux-riscv-2e094abfd1f29a08a60523b42d4508281b8dee0e.zip | |
ipc/sem.c: change memory barrier in sem_lock() to smp_rmb()
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>
Diffstat (limited to 'ipc')
| -rw-r--r-- | ipc/sem.c | 13 |
1 files changed, 10 insertions, 3 deletions
diff --git a/ipc/sem.c b/ipc/sem.c index 53c3310f41c6..6115146563f9 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. */ |