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Date: Tue, 11 Nov 2008 18:48:00 +0100
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] [patch] Fix block I/O hang.
References: <4919B46C.3040804@redhat.com> <4919B79A.5030402@codemonkey.ws>
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To: qemu-devel@nongnu.org

Anthony Liguori wrote:
> Gerd Hoffmann wrote:
>>   Hi,
>>
>> $subject says all.
>>
>> please apply,
>>   Gerd
> 
> Under what circumstances?  posix_aio_read() is only invoked from a
> select callback.  This means there should be data available to be read.

Hmm, looked at the loop again a bit closer.

First, the check for errno == EAGAIN is superfluous when the filehandle
isn't in non-blocking mode.  It will not happen.

Second, the check for errno == EINTR is superfluous too because the loop
will retry the read() syscall anyway for *any* error.

Third, when called from a select callback it shouldn't block indeed.  It
does though for me now and then when booting xen guests (with a big
stack of xenner patches).  Doesn't reproduce reliable though.  Sprinkled
in a printk (with rfd being non-blocking) and got a EAGAIN once, so it
got called with rfd not having data.

cheers,
  Gerd