summaryrefslogtreecommitdiff
path: root/ipc
diff options
context:
space:
mode:
authorManfred Spraul <manfred@colorfullife.com>2014-12-12 16:58:11 -0800
committerJiri Slaby <jslaby@suse.cz>2015-02-16 14:56:32 +0100
commitb116a0436c29bd83196d1988d75521044dc36e5d (patch)
treea278cd687932047f9f5120cf462f8895a558718e /ipc
parent23b4103455d52e2ffdc4758aea29ba6d70d246ca (diff)
ipc/sem.c: change memory barrier in sem_lock() to smp_rmb()
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> Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Diffstat (limited to 'ipc')
-rw-r--r--ipc/sem.c13
1 files changed, 10 insertions, 3 deletions
diff --git a/ipc/sem.c b/ipc/sem.c
index db9d241af133..0c312ac04e49 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.
*/