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

[Halil Pasic]

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: address@hidden
>> Signed-off-by: Halil Pasic <address@hidden>
>> Reported-by: Michael A. Tebolt <address@hidden>
>> Tested-by: Michael A. Tebolt <address@hidden>
>> Suggested-by: Paolo Bonzini <address@hidden>
>> ---
>>
>> 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.)

Regards,
Halil