[Xen-devel] [PATCH 1/2] xen: fix race in xen_qlock_wait()

[Juergen Gross <jgross@xxxxxxxx>]

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


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