linux-kernel - Re: [PATCH v2 3/8] xen/events: avoid handling the same event on two cpus at the same time

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Message-ID: <c4d930c1-331f-6a1e-7d26-cf066cecda33@suse.com>
Date:   Mon, 15 Feb 2021 07:55:14 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Julien Grall <julien@....org>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH v2 3/8] xen/events: avoid handling the same event on two
 cpus at the same time

On 14.02.21 22:34, Julien Grall wrote:
> Hi Juergen,
> 
> On 11/02/2021 10:16, Juergen Gross wrote:
>> When changing the cpu affinity of an event it can happen today that
>> (with some unlucky timing) the same event will be handled on the old
>> and the new cpu at the same time.
>>
>> Avoid that by adding an "event active" flag to the per-event data and
>> call the handler only if this flag isn't set.
>>
>> Reported-by: Julien Grall <julien@....org>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>> ---
>> V2:
>> - new patch
>> ---
>>   drivers/xen/events/events_base.c | 19 +++++++++++++++----
>>   1 file changed, 15 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/xen/events/events_base.c 
>> b/drivers/xen/events/events_base.c
>> index e157e7506830..f7e22330dcef 100644
>> --- a/drivers/xen/events/events_base.c
>> +++ b/drivers/xen/events/events_base.c
>> @@ -102,6 +102,7 @@ struct irq_info {
>>   #define EVT_MASK_REASON_EXPLICIT    0x01
>>   #define EVT_MASK_REASON_TEMPORARY    0x02
>>   #define EVT_MASK_REASON_EOI_PENDING    0x04
>> +    u8 is_active;        /* Is event just being handled? */
>>       unsigned irq;
>>       evtchn_port_t evtchn;   /* event channel */
>>       unsigned short cpu;     /* cpu bound */
>> @@ -622,6 +623,7 @@ static void xen_irq_lateeoi_locked(struct irq_info 
>> *info, bool spurious)
>>       }
>>       info->eoi_time = 0;
>> +    smp_store_release(&info->is_active, 0);
>>       do_unmask(info, EVT_MASK_REASON_EOI_PENDING);
>>   }
>> @@ -809,13 +811,15 @@ static void pirq_query_unmask(int irq)
>>   static void eoi_pirq(struct irq_data *data)
>>   {
>> -    evtchn_port_t evtchn = evtchn_from_irq(data->irq);
>> +    struct irq_info *info = info_for_irq(data->irq);
>> +    evtchn_port_t evtchn = info ? info->evtchn : 0;
>>       struct physdev_eoi eoi = { .irq = pirq_from_irq(data->irq) };
>>       int rc = 0;
>>       if (!VALID_EVTCHN(evtchn))
>>           return;
>> +    smp_store_release(&info->is_active, 0);
> 
> Would you mind to explain why you are using the release semantics?

It is basically releasing a lock. So release semantics seem to be
appropriate.

> It is also not clear to me if there are any expected ordering between 
> clearing is_active and clearing the pending bit.

No, I don't think there is a specific ordering required. is_active is
just guarding against two simultaneous IRQ handler calls for the same
event being active. Clearing the pending bit is not part of the guarded
section.

> 
>>       clear_evtchn(evtchn);
> 
> 
> The 2 lines here seems to be a common pattern in this patch. So I would 
> suggest to create a new helper.

Okay.


Juergen

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