Re: [RFC PATCH 0/5] Removal of AioContext lock, bs->parents and ->children: proof of concept

From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>
Cc: Fam Zheng <fam@euphon.net>, Kevin Wolf <kwolf@redhat.com>,
	Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	"open list:Block layer core" <qemu-block@nongnu.org>,
	qemu-devel <qemu-devel@nongnu.org>,
	Hanna Reitz <hreitz@redhat.com>, John Snow <jsnow@redhat.com>
Subject: Re: [RFC PATCH 0/5] Removal of AioContext lock, bs->parents and ->children: proof of concept
Date: Mon, 4 Apr 2022 11:51:51 +0200	[thread overview]
Message-ID: <7d82deaa-e34e-c3b4-0d1a-80fe9053c1a8@redhat.com> (raw)
In-Reply-To: <CABgObfZ96TOf9nxdrHrtKtrfyG0sZS9rPqAaReQgxNQ+AkKKpA@mail.gmail.com>

Am 04/04/2022 um 11:41 schrieb Paolo Bonzini:
> 
> 
> On Mon, Apr 4, 2022 at 11:25 AM Stefan Hajnoczi <stefanha@redhat.com
> <mailto:stefanha@redhat.com>> wrote:
> 
>     - The new API doesn't stop more I/O requests from being submitted, it
>       just blocks the current coroutine so request processing is deferred.
> 
> 
> New I/O requests would not complete until the write-side critical
> section ends. However they would still be accepted: from the point of
> view of the guest, the "consumed" index of the virtio ring would move
> forward, unlike bdrv_drained_begin/end().
> 
>     - In other words, is_external is a flow control API whereas the new API
>       queues up request coroutines without notifying the caller.
> 
> 
> Yes, I think this is the same I wrote above.
> 
>     - The new API still needs to be combined with bdrv_drained_begin/end()
>       to ensure in-flight requests are done.
> 
> 
> I don't think so, because in-flight requests would take the lock for
> reading. The write side would not start until those in-flight requests
> release the lock.
> 
>     - It's not obvious to me whether the new API obsoletes is_external.
>     I think it probably doesn't.
> 
> 
> I agree that it doesn't. This new lock is only protecting ->parents and
> ->children. 

Side note: it will also be used to protect other fields, like
.aio_context I think. I haven't checked if there is something else we
might want to protect that is currently protected by AioContext lock.

At least, I think we are going to use the same lock, right?

Emanuele

bdrv_drained_begin()/end() remains necessary, for example,
> when you need to send a request during the drained section. An example
> is block_resize.
> 
> In addition, bdrv_drained_begin()/end() ensures that the callback of
> blk_aio_*() functions has been invoked (see commit 46aaf2a566,
> "block-backend: Decrease in_flight only after callback", 2018-09-25). 
> This new lock would not ensure that.
> 
> As an aside, instead of is_external, QEMU could remove/add the ioeventfd
> handler in the blk->dev_ops->drained_begin and blk->dev_ops->drained_end
> callbacks respectively. But that's just a code cleanup.
> 
> Paolo