[PATCH 4.14 120/222] xen: fix race in xen_qlock_wait()

Greg Kroah-Hartman
2018-11-11T22:59:41Z

From: Juergen Gross <jgross@suse.com>

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@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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
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
@@ -46,17 +46,12 @@ 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;