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Re: [Qemu-devel] [RFC PATCH 0/4] aio: Don't poll ioeventfd in nested aio

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC PATCH 0/4] aio: Don't poll ioeventfd in nested aio_poll()
Date: Wed, 27 May 2015 11:38:57 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

On 27/05/2015 09:19, Fam Zheng wrote:
> This series looks at the other side of the broken "qmp transaction" problem
> with dataplane [1] - the event loop.
> Before, an ioeventfd of a non-dataplane device is registered via
> qemu_set_fd_handler, which is only polled directly by main_loop_wait(), not in
> aio_poll(); an ioeventfd of a dataplane device (virtio-blk/virtio-scsi) is
> registered specially via aio_set_event_notifier, which is a wrapper of
> aio_set_fd_handler, thus it WILL be polled by all aio_poll().
> As explained in [1], and in patch 3, this is wrong. The handlers mustn't run.
> [1] Fixes it by unregistering them temporarily around such nested poll, while
> this series fixes it by skipping those file descriptors in all nested
> aio_poll(), just like how iohandler behaves in the main loop.
> On the one hand, it is simpler than [1]; on the other hand, this approach is
> also interesting because once we remove qemu_set_fd_handler2 [2], iohandler.c
> can be removed by converting all qemu_set_fd_handler to the new
> aio_set_io_event_notifier introduced in this series.

I don't think this can work.

Whenever the main context is doing synchronous work for dataplane, it is
in the outermost aio_poll.


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