Re: [Qemu-devel] [PATCH v2 00/11] aio: Introduce handler type to fix nested aio_poll for dataplane

[Fam Zheng <famz@redhat.com>]

On Fri, 09/11 12:39, Kevin Wolf wrote:
> Am 29.07.2015 um 06:42 hat Fam Zheng geschrieben:
> > v2: Switch to disable/enable model. [Paolo]
> > 
> > Most existing nested aio_poll()'s in block layer are inconsiderate of
> > dispatching potential new r/w requests from ioeventfds and nbd exports, which
> > might result in responsiveness issues (e.g. bdrv_drain_all will not return when
> > new requests keep coming), or even wrong semantics (e.g. qmp_transaction cannot
> > enforce atomicity due to aio_poll in bdrv_drain_all).
> > 
> > Previous attampts to address this issue include new op blocker[1], bdrv_lock[2]
> > and nested AioContext (patches not posted to qemu-devel).
> > 
> > This approach is based on the idea proposed by Paolo Bonzini. The original idea
> > is introducing "aio_context_disable_client / aio_context_enable_client to
> > filter AioContext handlers according to the "client", e.g.
> > AIO_CLIENT_DATAPLANE (ioeventfd), AIO_CLIENT_PROTOCOL, AIO_CLIENT_NBD_SERVER,
> > AIO_CLIENT_CONTEXT, ... Extend aio_set_{event_notifier,fd}_handler to pass a
> > client (type)." 
> > 
> > After this series, block layer aio_poll() will only process those "protocol"
> > fds that are used in block I/O, plus the ctx->notifier for aio_notify();  other
> > aio_poll()'s keep unchanged.
> > 
> > The biggest advantage over approaches [1] and [2] is, no change is needed in
> > virtio-{blk,scsi}-dataplane code, also this doesn't depend on converting QMP to
> > coroutines.
> 
> It seems that I haven't replied on the mailing list yet, even though I
> think I already said this in person at KVM Forum: This series fixes only
> a special case of the real problem, which is that bdrv_drain/all at a
> single point doesn't make a lot of sense, but needs to be replaced by a
> whole section with exclusive access, like a bdrv_drained_begin/end pair.
> 
> To be clear: Anything that works with types of users instead of
> individual users is bound to fall short of being a complete solution. I
> don't prefer partial solutions when we know there is a bigger problem.
> 
> This series addresses your immediate need of protecting against new data
> plane requests, which it arguably achieves. The second case I always
> have in mind is Berto's case where he has multiple streaming block jobs
> in the same backing file chain [1].
> 
> This involves a bdrv_reopen() of the target BDS to make it writable, and
> bdrv_reopen() uses bdrv_drain_all() so drivers don't have to cope with
> running requests while reopening themselves. It can however involve
> nested event loops for synchronous operations like bdrv_flush(), and if
> those can process completions of block jobs, which can respond by doing
> anything to the respective node, things can go wrong.

Just to get a better idea of bdrv_drained_begin/end, could you explain how to
use the pair to fix the above problem?

> 
> You don't solve this by adding client types (then problematic request
> would be PROTOCOL in your proposal and you can never exclude that), but
> you really need to have bdrv_drained_being/end pairs, where only
> requests issued in between are processed and everything else waits.

What do you mean by "only requests issued in between are processed"? Where are
the requests from?

Fam