On Tue, 19 Mar 2013, David Vrabel wrote:
> 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
> delayed.
>
> Avoid this by always clearing the mask bit before checking for
> pending.
>
> 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.
The race you are trying to fix is real, but the fix you are proposing
breaks PV on HVM and ARM guests again.
>From the description of b5e579232d635b79a3da052964cb357ccda8d9ea, it's
clear that the reason to call EVTCHNOP_unmask is to trigger an event
notification injection again.
But if you clear the evtchn_mask bit *before* calling EVTCHNOP_unmask,
EVTCHNOP_unmask won't reinject the event.
>From evtchn_unmask:
if ( test_and_clear_bit(port, &shared_info(d, evtchn_mask)) &&
test_bit (port, &shared_info(d, evtchn_pending)) &&
!test_and_set_bit (port / BITS_PER_EVTCHN_WORD(d),
&vcpu_info(v, evtchn_pending_sel)) )
{
vcpu_mark_events_pending(v);
}
The first condition for reinjection would fail.
> 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
> --
> 1.7.2.5
>
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