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[Xen-devel] [PATCH] xen/events: avoid race with raising an event in unmask_evtchn()

From: David Vrabel <david.vrabel@xxxxxxxxxx>

In unmask_evtchn(), when the mask bit is cleared after testing for
pending and the event becomes pending between the test and clear, then
the upcall will not become pending and the event may be lost or

Avoid this by always clearing the mask bit before checking for

This fixes a regression introduced in 3.7 by
b5e579232d635b79a3da052964cb357ccda8d9ea (xen/events: fix
unmask_evtchn for PV on HVM guests) which reordered the clear mask and
check pending operations.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
 drivers/xen/events.c |   10 +++++-----
 1 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/xen/events.c b/drivers/xen/events.c
index d17aa41..4bdd0a5 100644
--- a/drivers/xen/events.c
+++ b/drivers/xen/events.c
@@ -403,11 +403,13 @@ static void unmask_evtchn(int port)
        if (unlikely((cpu != cpu_from_evtchn(port))))
                do_hypercall = 1;
-       else
+       else {
+               sync_clear_bit(port, BM(&s->evtchn_mask[0]));
                evtchn_pending = sync_test_bit(port, BM(&s->evtchn_pending[0]));
-       if (unlikely(evtchn_pending && xen_hvm_domain()))
-               do_hypercall = 1;
+               if (unlikely(evtchn_pending && xen_hvm_domain()))
+                       do_hypercall = 1;
+       }
        /* Slow path (hypercall) if this is a non-local port or if this is
         * an hvm domain and an event is pending (hvm domains don't have
@@ -418,8 +420,6 @@ static void unmask_evtchn(int port)
        } else {
                struct vcpu_info *vcpu_info = __this_cpu_read(xen_vcpu);
-               sync_clear_bit(port, BM(&s->evtchn_mask[0]));
                 * The following is basically the equivalent of
                 * 'hw_resend_irq'. Just like a real IO-APIC we 'lose

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