Re: [PATCH 1/8] virtio_queue_aio_attach_host_notifier: remove AioContext lock

[Emanuele Giuseppe Esposito]

Am 05/07/2022 um 16:11 schrieb Stefan Hajnoczi:
> On Thu, Jun 09, 2022 at 10:37:20AM -0400, Emanuele Giuseppe Esposito wrote:
>> @@ -146,7 +147,6 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
>>  
>>      s->dataplane_starting = false;
>>      s->dataplane_started = true;
>> -    aio_context_release(s->ctx);
>>      return 0;
> 
> This looks risky because s->dataplane_started is accessed by IO code and
> there is a race condition here. Maybe you can refactor the code along
> the lines of virtio-blk to avoid the race.
> 

Uhmm could you explain why is virtio-blk also safe here?
And what is currently protecting dataplane_started (in both blk and
scsi, as I don't see any other AioContext lock taken)?

Because I see that for example virtio_blk_req_complete is IO_CODE, so it
could theoretically read dataplane_started while it is being changed in
dataplane_stop? Even though I guess it doesn't because we disable and
clean the host notifier before modifying it?

But if so, I don't get what is the difference with scsi code, and why we
need to protect only that instance with the aiocontext lock?

Thank you,
Emanuele