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Re: [Qemu-devel] [Qemu-block] [PATCH v3 2/2] aio: Do aio_notify_accept o

From: Kevin Wolf
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v3 2/2] aio: Do aio_notify_accept only during blocking aio_poll
Date: Fri, 7 Sep 2018 17:51:01 +0200
User-agent: Mutt/1.9.1 (2017-09-22)

Am 09.08.2018 um 15:22 hat Fam Zheng geschrieben:
> Furthermore, blocking aio_poll is only allowed on home thread
> (in_aio_context_home_thread), because otherwise two blocking
> aio_poll()'s can steal each other's ctx->notifier event and cause
> hanging just like described above.

It's good to have this assertion now at least, but after digging into
some bugs, I think in fact that any aio_poll() (even non-blocking) is
only allowed in the home thread: At least one reason is that if you run
it from a different thread, qemu_get_current_aio_context() returns the
wrong AioContext in any callbacks called by aio_poll(). Anything else
using TLS can have similar problems.

One instance where this matters is fixed/worked around by Sergio's
"util/async: use qemu_aio_coroutine_enter in co_schedule_bh_cb". We
wouldn't even need that patch if we could make sure that aio_poll() is
never called from the wrong thread. This would feel more robust.

I'll fix the aio_poll() calls in drain (the AIO_WAIT_WHILE() ones are
already fine, the rest by removing them). After that,
bdrv_set_aio_context() is still problematic, but the rest should be
okay. Hopefully we can use the tighter assertion then.


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