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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

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.

Paolo