[PATCH 3.19.y-ckt 58/92] x86/spinlocks: Fix regression in spinlock contention detection

[Kamal Mostafa]

3.19.8-ckt1 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Tahsin Erdogan <tahsin at google.com>

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.

Fixes: d6abfdb20223 ("x86/spinlocks/paravirt: Fix memory corruption on unlock")
Signed-off-by: Tahsin Erdogan <tahsin at google.com>
Cc: peterz at infradead.org
Cc: Waiman.Long at hp.com
Cc: borntraeger at de.ibm.com
Cc: oleg at redhat.com
Cc: raghavendra.kt at linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1430799331-20445-1-git-send-email-tahsin@google.com
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/x86/include/asm/spinlock.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index cf87de3..64b6117 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -169,7 +169,7 @@ static inline int arch_spin_is_contended(arch_spinlock_t *lock)
 	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
 
-- 
1.9.1