Re: [Qemu-devel] [RFC PATCH] fix select(2) race between main_loop_wait and qemu_aio_wait

[Paolo Bonzini <pbonzini@redhat.com>]

Il 05/03/2012 15:24, Avi Kivity ha scritto:
> On 03/05/2012 11:07 AM, Jan Kiszka wrote:
>> On 2012-03-05 09:34, Paolo Bonzini wrote:
>>> This is quite ugly.  Two threads, one running main_loop_wait and
>>> one running qemu_aio_wait, can race with each other on running the
>>> same iohandler.  The result is that an iohandler could run while the
>>> underlying socket is not readable or writable, with possibly ill effects.
>>
>> Hmm, isn't it a problem already that a socket is polled by two threads
>> at the same time? Can't that be avoided?
> 
> Could it be done simply by adding a mutex there?  It's hardly a clean
> fix, but it's not a clean problem.

Hmm, I don't think so.  It would need to protect execution of the
iohandlers too, and pretty much everything can happen there including a
nested loop.  Of course recursive mutexes exist, but it sounds like too
big an axe.

I could add a generation count updated by qemu_aio_wait(), and rerun the
select() only if the generation count changes during its execution.

Or we can call it an NBD bug.  I'm not against that, but it seemed to me
that the problem is more general.

Paolo