x86/spinlocks: Fix regression in spinlock contention detection

Tahsin Erdogan · KAGA-KOKO · commit e8a4a2696fec · 2015-05-05T11:01:38.000+02:00
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: d6abfdb ("x86/spinlocks/paravirt: Fix memory corruption on unlock") Signed-off-by: Tahsin Erdogan <tahsin@google.com> Cc: peterz@infradead.org Cc: Waiman.Long@hp.com Cc: borntraeger@de.ibm.com Cc: oleg@redhat.com Cc: raghavendra.kt@linux.vnet.ibm.com Cc: stable@vger.kernel.org Link: http://lkml.kernel.org/r/1430799331-20445-1-git-send-email-tahsin@google.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
diff --git 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
 

Original file line number	Diff line number	Diff line change
`@@ -169,7 +169,7 @@ static inline int arch_spin_is_contended(arch_spinlock_t *lock)`
`169`	`169`	`struct __raw_tickets tmp = READ_ONCE(lock->tickets);`
`170`	`170`
`171`	`171`	`tmp.head &= ~TICKET_SLOWPATH_FLAG;`
`172`		`- return (tmp.tail - tmp.head) > TICKET_LOCK_INC;`
	`172`	`+ return (__ticket_t)(tmp.tail - tmp.head) > TICKET_LOCK_INC;`
`173`	`173`	`}`
`174`	`174`	`#define arch_spin_is_contended arch_spin_is_contended`
`175`	`175`