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

On Thu, 9 Aug 2012, Nicolas Pitre wrote:

On Thu, 9 Aug 2012, Will Deacon wrote:

quoted

I think we could actually fix this entirely in mutex-xchg.h by doing
something in fastpath_lock similar to what we do for trylock:

diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h
index 580a6d3..c082e99 100644
--- a/include/asm-generic/mutex-xchg.h
+++ b/include/asm-generic/mutex-xchg.h

@@ -25,8 +25,19 @@
 static inline void
 __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
 {
-       if (unlikely(atomic_xchg(count, 0) != 1))
-               fail_fn(count);
+       int prev = atomic_xchg(count, 0);
+
+       if (unlikely(prev != 1)) {
+               if (prev < 0)
+                       /*
+                        * The lock was contended, so we need to restore
+                        * its original state to ensure that any waiting
+                        * tasks are woken up by the unlock slow path.
+                        */
+                       prev = atomic_xchg(count, prev);
+               if (prev != 1)
+                       fail_fn(count);
+       }
 }

What do you reckon?

Yes, that looks fine.  I'd remove that if (prev < 0) entirely though.  
We'll just swap a 0 for a 0 if the count wasn't < 0, or a 0 for a 1 if 
the mutex just got unlocked which is also fine.  This is especially 
beneficial when a native xchg processor instruction is used.

In fact, this suggestion isn't entirely correct either. The inner xchg 
in this case should be -1 and not 'count'.  If 'count' is 0 and the 
mutex becomes contended in the small window between the two xchg's then 
the contention mark would be lost again.

In other words, I think this should look like this:

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);
+	}
 }
 
 /**

Nicolas