Re: [Qemu-devel] [PATCH v10 01/16] block: Pause all jobs during bdrv_reopen_multiple()

[Kevin Wolf <kwolf@redhat.com>]

Am 11.10.2016 um 11:54 hat Paolo Bonzini geschrieben:
> On 11/10/2016 11:39, Kevin Wolf wrote:
> > Am 10.10.2016 um 18:41 hat Paolo Bonzini geschrieben:
> >> On 10/10/2016 17:37, Kevin Wolf wrote:
> >>>> +    while ((job = block_job_next(job))) {
> >>>> +        AioContext *aio_context = blk_get_aio_context(job->blk);
> >>>> +
> >>>> +        aio_context_acquire(aio_context);
> >>>> +        block_job_pause(job);
> >>>> +        aio_context_release(aio_context);
> >>>> +    }
> >>>> +
> >>>>      bdrv_drain_all();
> >>>
> >>> We already have a bdrv_drain_all() here, which does the same thing (and
> >>> more) internally, except that it resumes all jobs before it returns.
> >>> Maybe what we should do is split bdrv_drain_all() in a begin/end pair,
> >>> too.
> >>
> >> Hey, haven't I just suggested the same? :)  I swear I hadn't read this
> >> before.
> >>
> >>> The other point I'm wondering now is whether bdrv_drain_all() should
> >>> have the aio_disable/enable_external() pair that bdrv_drain() has.
> >>
> >> bdrv_drain_all need not have it, but its start/end replacement should.
> > 
> > Doesn't need it because it holds the AioContext lock?
> 
> No, because as soon as bdrv_drain_all exits, external file descriptors
> can be triggered again so I don't think the aio_disable/enable_external
> actually provides any protection.

Right, what I was thinking of was more like new requests coming in
while bdrv_drain_all() is still running. If that happened, the result
after bdrv_drain_all() wouldn't be different, but if external file
descriptors are still active and the guest keeps sending requests, it
could take forever until it returns.

But that part is addressed by the AioContext lock, I think.

> bdrv_drain_all should really only be used in cases where you already
> have some kind of "hold" on external file descriptors, like bdrv_close
> uses bdrv_drain():
> 
>     bdrv_drained_begin(bs); /* complete I/O */
>     bdrv_flush(bs);
>     bdrv_drain(bs); /* in case flush left pending I/O */
> 
> That said, because the simplest implementation of bdrv_drain_all() does
> 
>     bdrv_drained_all_start();
>     bdrv_drained_all_end();
> 
> just like bdrv_drain() does it, it's not a very interesting point.
> bdrv_drain_all will probably disable/enable external file descriptors,
> it just doesn't need that.

Yes, of course.

Kevin