Re: [Qemu-devel] [PATCH] scsi: always call notifier on async cancellation

[Paolo Bonzini]

On 17/12/2015 02:15, Fam Zheng wrote:
>> >      if (notifier) {
>> >          notifier_list_add(&req->cancel_notifiers, notifier);
>> >      }
>> > -    if (req->io_canceled) {
>> > -        return;
>> > -    }
>> >      scsi_req_ref(req);
>> >      scsi_req_dequeue(req);
>> >      req->io_canceled = true;
>        if (req->aiocb) {
>            blk_aio_cancel_async(req->aiocb);
>        } else {
>            scsi_req_cancel_complete(req);
>        }
> 
> A second TMF must be blk_aio_cancel_async case, otherwise the first one would
> have already completed the request synchronously in scsi_req_cancel_complete.

Good point.

> With that in mind, I think returning early is not a problem. But I suppose
> these are also idempotent so this change is not breaking anything, either.

Right, the issue is that all these calls are idempotent, but the
notifier may not; that is why I prefer to be safe and ensure that all
notifier additions are matched by a notify.  But you explained well why
this should be safe, I'll add a note to the commit message.

Paolo