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xen: fix race in xen_qlock_wait()

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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>
This commit is contained in:
Juergen Gross 2018-10-01 07:57:42 +02:00
parent af320de90e
commit 2ac2a7d4d9
1 changed files with 5 additions and 10 deletions
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15
arch/x86/xen/spinlock.c
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@ -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;