Patch 2 of my fast lock series actually also contained an attempt to fix
a race I spotted in the code that atomically sets the claimed bit. I
forgot about this fact and even, worse, I only replace the original race
with another one.
So here comes a new attempt to fix the issue that the lock owner and/or
the claimed bit can change between trylock and the cmpxchg under nklock.
Please have a look and cross-check the logic.
The patch applies on top of vanilla SVN, so my series has to be rebased
and the fix has to be ported to native as well - where we just found it
in the field.
Jan
---
ksrc/skins/posix/mutex.h | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)
Index: b/ksrc/skins/posix/mutex.h
===================================================================
--- a/ksrc/skins/posix/mutex.h
+++ b/ksrc/skins/posix/mutex.h
@@ -134,17 +134,18 @@ static inline int pse51_mutex_timedlock_
/* Set bit 0, so that mutex_unlock will know that the mutex is claimed.
Hold the nklock, for mutual exclusion with slow mutex_unlock. */
xnlock_get_irqsave(&nklock, s);
+ owner = xnarch_atomic_intptr_get(mutex->owner);
while(!test_claimed(owner)) {
- old = xnarch_atomic_intptr_cmpxchg(mutex->owner,
- owner, set_claimed(owner,
1));
- if (likely(old == owner))
- break;
- if (old == NULL) {
+ if (owner == NULL) {
/* Owner called fast mutex_unlock
(on another cpu) */
xnlock_put_irqrestore(&nklock, s);
goto retry_lock;
}
+ old = xnarch_atomic_intptr_cmpxchg(mutex->owner,
+ owner, set_claimed(owner,
1));
+ if (likely(old == owner))
+ break;
owner = old;
}
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