xen: fix race in xen_qlock_wait()

In the following situation a vcpu waiting for a lock might not be
woken up from xen_poll_irq():

CPU 1:                CPU 2:                      CPU 3:
takes a spinlock
                      tries to get lock
                      -> xen_qlock_wait()
frees the lock
-> xen_qlock_kick(cpu2)
                        -> xen_clear_irq_pending()

takes lock again
                                                  tries to get lock
                                                  -> *lock = _Q_SLOW_VAL
                        -> *lock == _Q_SLOW_VAL ?
                        -> xen_poll_irq()
frees the lock
-> xen_qlock_kick(cpu3)

And cpu 2 will sleep forever.

This can be avoided easily by modifying xen_qlock_wait() to call
xen_poll_irq() only if the related irq was not pending and to call
xen_clear_irq_pending() only if it was pending.

Cc: stable@vger.kernel.org
Cc: Waiman.Long@hp.com
Cc: peterz@infradead.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

arch/x86/xen/spinlock.c

@@ -45,17 +45,12 @@ static void xen_qlock_wait(u8 *byte, u8 val)
 	if (irq == -1)
 		return;
 
-	/* clear pending */
+	/* If irq pending already clear it and return. */
+	if (xen_test_irq_pending(irq)) {
 		xen_clear_irq_pending(irq);
-	barrier();
+		return;
+	}
 
-	/*
-	 * We check the byte value after clearing pending IRQ to make sure
-	 * that we won't miss a wakeup event because of the clearing.
-	 *
-	 * The sync_clear_bit() call in xen_clear_irq_pending() is atomic.
-	 * So it is effectively a memory barrier for x86.
-	 */
 	if (READ_ONCE(*byte) != val)
 		return;