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

[Kevin Wolf <kwolf@redhat.com>]

Am 11.09.2015 um 13:46 hat Fam Zheng geschrieben:
> 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?

How to use it is easy part: In bdrv_reopen_multiple(), you would replace
the existing bdrv_drain_all() with begin and you would add the
corresponding end right before the return statement.

> > 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?

Generally speaking, you would have code that looks like this:

    bdrv_drain_begin()
    ...
    bdrv_something_synchronous()
    ...
    bdrv_drain_end()

You want to process everything that is necessary for completing
bdrv_something_synchronous(), but nothing else.

The trickier question is how to implement this. I know that it's much
easier to say that your series doesn't work than actually proposing
something else that works...

One relatively obvious answer we found when we discussed this a while
back was some kind of a recursive CoRwLock (reader = in-flight request;
writer = drained section), but that requires obviously that you're
running in a coroutine if you want to do something with a drained
request queue.

I'm also not totally happy with the requirement of taking a reader lock
more or less everywhere. But I'm not sure yet if there is a good
alternative that can achieve the same.

This needs some more thought, I guess.

Kevin