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Date: Mon, 19 Aug 2013 15:21:14 +0200
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Subject: Re: [Qemu-devel] Using aio_poll for timer carrier threads
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To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, liu ping fan <qemulist@gmail.com>, Alex Bligh <alex@alex.org.uk>, MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>, Richard Henderson <rth@twiddle.net>

Il 13/08/2013 16:54, Jan Kiszka ha scritto:
>> > Using an AioContext lock for timers is somewhat complicated for lock
>> > ordering, because context A could try to modify a timer from context B,
>> > at the same time when context B is modifying a timer from context A.
>> > This would cause a deadlock.
> That's like MMIO access on device A triggers MMIO access on B and vice
> versa - why should we need this, so why should we support this? I think
> the typical case is that timers (and their lists) and data structures
> they access have a fairly close relation, thus can reuse the same lock.

Yes, that's true.  Still it would have to be documented, and using
too-coarse locks risks having many BQLs, which multiplies the complexity
(fine-grained locking at least keeps critical sections small and limits
the amount of nested locking).

I like Stefan's patches to make the timer list thread-safe, especially
if we can optimize it (with RCU?) to make the read side lockless.

Paolo