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()
-> xen_clear_irq_pending()
frees the lock
-> xen_qlock_kick(cpu2)
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>
---
arch/x86/xen/spinlock.c | 15 +++++----------
1 file changed, 5 insertions(+), 10 deletions(-)
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 973f10e05211..cd210a4ba7b1 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -45,17 +45,12 @@ static void xen_qlock_wait(u8 *byte, u8 val)
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;
--
2.16.4
