Patch "x86/spinlocks: Fix regression in spinlock contention detection" has been added to the 4.0-stable tree

This is a note to let you know that I've just added the patch titled

x86/spinlocks: Fix regression in spinlock contention detection

to the 4.0-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-spinlocks-fix-regression-in-spinlock-contention-detection.patch
and it can be found in the queue-4.0 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Tahsin Erdogan <[email protected]>
Date: Mon, 4 May 2015 21:15:31 -0700
Subject: x86/spinlocks: Fix regression in spinlock contention detection

From: Tahsin Erdogan <[email protected]>

commit e8a4a2696fecb398b0288c43c0e0dbb91e265bb2 upstream.

A spinlock is regarded as contended when there is at least one waiter. Currently, the code that checks whether there are any waiters rely on tail value being greater than head. However, this is not true if tail reaches the max value and wraps back to zero, so arch_spin_is_contended() incorrectly returns 0 (not contended) when tail is smaller than head.

The original code (before regression) handled this case by casting the (tail - head) to an unsigned value. This change simply restores that behavior.

---
 arch/x86/include/asm/spinlock.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -169,7 +169,7 @@ static inline int arch_spin_is_contended
 	struct __raw_tickets tmp = READ_ONCE(lock->tickets);
 
 	tmp.head &= ~TICKET_SLOWPATH_FLAG;
-	return (tmp.tail - tmp.head) > TICKET_LOCK_INC;
+	return (__ticket_t)(tmp.tail - tmp.head) > TICKET_LOCK_INC;
 }
 #define arch_spin_is_contended	arch_spin_is_contended
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.0/x86-spinlocks-fix-regression-in-spinlock-contention-detection.patch