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Re: [Qemu-devel] [PATCH v2 2/2] QEMUBH: make AioContext's bh re-entrant

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v2 2/2] QEMUBH: make AioContext's bh re-entrant
Date: Tue, 18 Jun 2013 21:20:26 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6

Il 18/06/2013 17:14, mdroth ha scritto:
> Could we possibly simplify this by introducing a recursive mutex that we
> could use to protect the whole list loop and hold even during the cb?

If it is possible, we should avoid recursive locks.  It makes impossible
to establish a lock hierarchy.  For example:

> I assume we can't hold the lock during the cb currently since we might
> try to reschedule, but if it's a recursive mutex would that simplify
> things?

If you have two callbacks in two different AioContexts, both of which do
bdrv_drain_all(), you get an AB-BA deadlock

Also, the barriers for qemu_bh_schedule are needed anyway.  It's only
those that order bh->next vs. ctx->first_bh that would go away.


> I've been doing something similar with IOHandlers for the QContext
> stuff, and that's the approach I took. This patch introduces the
> recursive mutex:
> https://github.com/mdroth/qemu/commit/c7ee0844da62283c9466fcb10ddbfadd0b8bfc53

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