xen/pvcalls: fix potential endless loop in pvcalls-front.c

mutex_trylock() returns 1 if you take the lock and 0 if not. Assume you
take in_mutex on the first try, but you can't take out_mutex. Next times
you call mutex_trylock() in_mutex is going to fail. It's an endless
loop.

Solve the problem by waiting until the global refcount is 1 instead (the
refcount is 1 when the only active pvcalls frontend function is
pvcalls_front_release).

Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: boris.ostrovsky@xxxxxxxxxx
CC: jgross@xxxxxxxx


diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
index 0c1ec68..72a74c3 100644
--- a/drivers/xen/pvcalls-front.c
+++ b/drivers/xen/pvcalls-front.c
@@ -1048,8 +1048,7 @@ int pvcalls_front_release(struct socket *sock)
 		 * is set to NULL -- we only need to wait for the existing
 		 * waiters to return.
 		 */
-		while (!mutex_trylock(&map->active.in_mutex) ||
-			   !mutex_trylock(&map->active.out_mutex))
+		while (atomic_read(&pvcalls_refcount) > 1)
 			cpu_relax();
 
 		pvcalls_front_free_map(bedata, map);