Re: [Xen-devel] [PATCH 01/12] xen/events: avoid race with raising an event in unmask_evtchn()

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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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