Hi Oleg,
On 02/26/2015 08:29 PM, Oleg Nesterov wrote:
@@ -341,7 +359,13 @@ static inline int sem_lock(struct sem_array *sma, struct
sembuf *sops,
* Thus: if is now 0, then it will stay 0.
*/
if (sma->complex_count == 0) {
- /* fast path successful! */
+ /*
+ * Fast path successful!
+ * We only need a final memory barrier.
+ * (see sem_wait_array() for details).
+ */
+ smp_rmb();
+
I'll try to read this again tomorrow, but so far I am confused.
Most probably I missed something, but this looks unneeded at first glance.
No, my fault:
I thought long about sem_wait_array() and then I did copy&paste without
thinking properly.
The sequence is:
thread A:
spin_lock(&local)
thread B:
complex_count=??;
spin_unlock(&global); <<< release_mb
thread A:
spin_unlock_wait(&global); <<< control_mb
smb_mb__after_control_barrier(); <<< acquire_mb
<<< now everything from thread B is visible.
<<< and: thread B has dropped the lock, it can't change any
protected var
<<< and: a new thread C can't acquire a lock, we hold &local.
if (complex_count == 0) goto success;
I'll update the patch.
(cc stable, starting from 3.10...)
--
Manfred
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