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 <[email protected]>
---
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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