Re: [Qemu-devel] [PATCH v2 1/1] virtio-blk: fix race on guest notifiers

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/03/2017 13:41, Halil Pasic wrote:
> 
> 
> On 03/10/2017 10:08 PM, Halil Pasic wrote:
>>
>>
>> On 03/10/2017 05:47 PM, Paolo Bonzini wrote:
>>>
>>> On 07/03/2017 14:16, Halil Pasic wrote:
>>>> The commits 03de2f527 "virtio-blk: do not use vring in dataplane"  and
>>>> 9ffe337c08 "virtio-blk: always use dataplane path if ioeventfd is active"
>>>> changed how notifications are done for virtio-blk substantially. Due to a
>>>> race condition, interrupts are lost when irqfd behind the guest notifier
>>>> is torn down after notify_guest_bh was scheduled but before it actually
>>>> runs.
>>>>
>>>> Let's fix this by forcing guest notifications before cleaning up the
>>>> irqfd's. Let's also add some explanatory comments.
>>>>
>>>> Cc: qemu-stable@nongnu.org
>>>> Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
>>>> Reported-by: Michael A. Tebolt <miket@us.ibm.com>
>>>> Tested-by: Michael A. Tebolt <miket@us.ibm.com>
>>>> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
>>>> ---
>>>>
>>>> This patch withstood the test case which discovered the problem
>>>> for several days (as reported by Michale Tebolt).
>>>>
>>>> v1 --> v2:
>>>> * Fixed typo pointed out by Connie
>>>> * Added Tested-by
>>> Hi Halil,
>>>
>>> I found a similar issue in NBD.  Can you check if this patch fixes
>>> the virtio-blk issue too?
>>>
>>> Thanks,
>>> Paolo
>>>
>>> ------ 8< ------------
>>>
>>> diff --git a/block.c b/block.c
>>> index f293ccb..e159251 100644
>>> --- a/block.c
>>> +++ b/block.c
>>> @@ -4272,8 +4272,15 @@ void bdrv_attach_aio_context(BlockDriverState *bs,
>>>
>>>  void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context)
>>>  {
>>> +    AioContext *ctx;
>>> +
>>>      bdrv_drain(bs); /* ensure there are no in-flight requests */
>>>
>>> +    ctx = bdrv_get_aio_context(bs);
>>> +    while (aio_poll(ctx, false)) {
>>> +        /* wait for all bottom halves to execute */
>>> +    }
>>> +
>>>      bdrv_detach_aio_context(bs);
>>>
>>>      /* This function executes in the old AioContext so acquire the new one in
>>>
>>>
>>
>> So far so good! I will let it spin over the weekend but I think it's unlikely
>> something will turn up.
>>
>> I was wondering, would it make sense to push this logic into bdrv_drain?
>> (Along the lines: this looks much like tying up loose ends drain has left.
>> But I'm not sure about it.)
>>
> 
> I think it's safe to say that this fixes the virtio-blk issue too. Are you
> going to send a proper patch with this (or an equivalent) change?

Yes, I am, thanks!

Paolo