Re: [Qemu-devel] [Qemu-block] [PATCH v3 2/2] aio: Do aio_notify_accept only during blocking aio_poll

[Fam Zheng <famz@redhat.com>]

On Fri, 09/07 17:51, Kevin Wolf wrote:
> 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.

Fully agree with you.

Fam