[PATCH 3.12 102/122] ipc/sem.c: change memory barrier in sem_lock() to smp_rmb()

From: Jiri Slaby
Date: Tue Feb 17 2015 - 06:45:57 EST

From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>

3.12-stable review patch. If anyone has any objections, please let me know.


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@xxxxxxxxxxxxxxxxxxxx: update sem_lock() comment, from Davidlohr]
Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Acked-by: Rafael Aquini <aquini@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
ipc/sem.c | 13 ++++++++++---
1 file 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.

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