[PATCH 4.9 079/141] xen: fix race in xen_qlock_wait()

[Greg Kroah-Hartman]

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

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

From: Juergen Gross <jgross@xxxxxxxx>

commit 2ac2a7d4d9ff4e01e36f9c3d116582f6f655ab47 upstream.

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@xxxxxxxxxxxxxxx
Cc: Waiman.Long@xxxxxx
Cc: peterz@xxxxxxxxxxxxx
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/x86/xen/spinlock.c |   15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -45,17 +45,12 @@ static void xen_qlock_wait(u8 *byte, u8
 	if (irq == -1)
 		return;
 
-	/* clear pending */
-	xen_clear_irq_pending(irq);
-	barrier();
+	/* If irq pending already clear it and return. */
+	if (xen_test_irq_pending(irq)) {
+		xen_clear_irq_pending(irq);
+		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;