RFC: mutex: hung tasks on SMP platforms with asm-generic/mutex-xchg.h

On Thu, Aug 09, 2012 at 07:09:02PM +0100, Nicolas Pitre wrote:

On Thu, 9 Aug 2012, Will Deacon wrote:

quoted

On Thu, Aug 09, 2012 at 05:57:33PM +0100, Nicolas Pitre wrote:

quoted

On Thu, 9 Aug 2012, Nicolas Pitre wrote:

diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h
index 580a6d35c7..44a66c99c8 100644
--- a/include/asm-generic/mutex-xchg.h
+++ b/include/asm-generic/mutex-xchg.h

@@ -25,8 +25,11 @@
 static inline void
 __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
 {
-	if (unlikely(atomic_xchg(count, 0) != 1))
-		fail_fn(count);
+	if (unlikely(atomic_xchg(count, 0) != 1)) {
+		/* Mark lock contention explicitly */
+		if (likely(atomic_xchg(count, -1) != 1))
+			fail_fn(count);
+	}
 }
 
 /**

Doesn't this mean that we're no longer just swapping 0 for a 0 if the lock
was taken, therefore needlessly sending the current owner down the slowpath
on unlock?

If the lock was taken, this means the count was either 0 or -1.  If it 
was 1 then we just put a 0 there and we own it.  But if the cound was 0 
then we should store -1 instead, which is what the inner xchg does.  If 
the count was already -1 then we store -1 back.  That more closely mimic 
what the atomic dec does which is what we want.

Ok, I just wasn't sure that marking the lock contended was required when it
was previously locked, given that we'll drop into spinning on the owner
anyway.

I'll add a commit message to the above and re-post if that's ok?

Will