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Date: Thu, 1 Aug 2013 15:51:11 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC v2 3/5] timer: make qemu_clock_enable sync
 between disable and timer's cb
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To: Alex Bligh <alex@alex.org.uk>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Jan Kiszka <jan.kiszka@siemens.com>, liu ping fan <qemulist@gmail.com>, qemu-devel@nongnu.org, Anthony Liguori <anthony@codemonkey.ws>

On Aug 01 2013, Alex Bligh wrote:
> >So actually there is another problem with this patch (both the
> >condvar and the event approach are equally buggy).  If a timer
> >on clock X disables clock X, qemu_clock_enable will deadlock.
> 
> Yes. I believe there will be a similar problem if a timer
> created or deleted an AioContext (clearly less likely!)
> 
> >I suppose it's easier to ask each user of qemu_clock_enable to
> >provide its own synchronization, and drop this patch.  The simplest
> >kind of synchronization is to delay some work to a bottom half in
> >the clock's AioContext.  If you do that, you know that the timers
> >are not running.
> 
> I'm not sure that's true. If two AioContexts run in different
> threads, would their BH's and timers not also run in those two
> different threads?

Suppose a thread wants to do qemu_clock_enable(foo, false), and the
code after qemu_clock_enable assumes that no timers are running.
Then you should move that code to a bottom half in foo's AioContext.

Paolo