locking/pvqspinlock: Fix kernel panic in locking-selftest

Enabling locking-selftest in a VM guest may cause the following
kernel panic:

  kernel BUG at .../kernel/locking/qspinlock_paravirt.h:137!

This is due to the fact that the pvqspinlock unlock function is
expecting either a _Q_LOCKED_VAL or _Q_SLOW_VAL in the lock
byte. This patch prevents that bug report by ignoring it when
debug_locks_silent is set. Otherwise, a warning will be printed
if it contains an unexpected value.

With this patch applied, the kernel locking-selftest completed
without any noise.

Tested-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Waiman Long <Waiman.Long@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1436663959-53092-1-git-send-email-Waiman.Long@hp.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/qspinlock_paravirt.h

@@ -4,6 +4,7 @@
 
 #include <linux/hash.h>
 #include <linux/bootmem.h>
+#include <linux/debug_locks.h>
 
 /*
  * Implement paravirt qspinlocks; the general idea is to halt the vcpus instead
@@ -286,15 +287,23 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock)
 {
 	struct __qspinlock *l = (void *)lock;
 	struct pv_node *node;
+	u8 lockval = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);
 
 	/*
 	 * We must not unlock if SLOW, because in that case we must first
 	 * unhash. Otherwise it would be possible to have multiple @lock
 	 * entries, which would be BAD.
 	 */
-	if (likely(cmpxchg(&l->locked, _Q_LOCKED_VAL, 0) == _Q_LOCKED_VAL))
+	if (likely(lockval == _Q_LOCKED_VAL))
 		return;
 
+	if (unlikely(lockval != _Q_SLOW_VAL)) {
+		if (debug_locks_silent)
+			return;
+		WARN(1, "pvqspinlock: lock %p has corrupted value 0x%x!\n", lock, atomic_read(&lock->val));
+		return;
+	}
+
 	/*
 	 * Since the above failed to release, this must be the SLOW path.
 	 * Therefore start by looking up the blocked node and unhashing it.