Commit 2ac2a7d4 authored Oct 01, 2018 by Juergen Gross

xen: fix race in xen_qlock_wait()



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>

parent af320de9

arch/x86/xen/spinlock.c

+5 −10

Original line number	Diff line number	Diff line
		@@ -45,17 +45,12 @@ static void xen_qlock_wait(u8 *byte, u8 val)
		if (irq == -1)
		return;

		/* clear pending */
		/* If irq pending already clear it and return. */
		if (xen_test_irq_pending(irq)) {
		xen_clear_irq_pending(irq);
		barrier();
		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;

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