Re: [Qemu-block] [Qemu-devel] [PATCH v4 13/13] block/mirror: Block "device IO" during mirror exit

[Paolo Bonzini]

On 19/05/2015 20:37, Fam Zheng wrote:
> On Tue, 05/19 10:49, Paolo Bonzini wrote:
>>
>>
>> On 19/05/2015 18:48, Fam Zheng wrote:
>>>>>
>>>>> This is too late.  As a rule, the blocker must be established before
>>>>> calling bdrv_drain, and removed on the next yield (in this case, before
>>>>> the assignment to last_pause_ns).
>>> I don't understand. If the blocker is removed before mirror_run returns,
>>> wouldn't more device IO already hit source image by the time mirror_exit 
>>> runs?
>>
>> If you go to mirror_exit, you won't reach the assignment (so you have to
>> remove the blocker in mirror_exit too).
>>
>> But if you don't go to mirror_exit because cnt != 0, you must remove the
>> blocker before the next I/O.
>>
> 
> OK, but I'm still not clear how is it too late in this patch? Although the
> blocker is set after bdrv_drain, we know there is no dirty data because cnt is
> 0, and we'll be holding a blocker when releasing the AioContext, no new IO is
> allowed.

So you rely on the caller of mirror_run not calling aio_context_release
between bdrv_drain and block_job_defer_to_main_loop?  That indeed should
work, but why not stick to a common pattern of blocking I/O before
bdrv_drain?  That's how bdrv_drain is almost always used in the code, so
it's a safe thing to do and the preemption points are then documented
more clearly.

I think it would be nice to have all bdrv_drain calls:

- either preceded by a device I/O blocker

- or preceded by a comment explaining why the call is there and why it
doesn't need the blocker

This is not a NACK, but I would like to understand the disadvantages of
what I am suggesting here.

Paolo