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Date: Thu, 25 Sep 2008 11:00:25 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] Re: [PATCH]: fix QEMU SCSI lock up
References: <20080924225946.GA28588@dmt.cnet> <48DACF6F.40301@us.ibm.com>
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To: qemu-devel@nongnu.org
Cc: Marcelo Tosatti <mtosatti@redhat.com>

Anthony Liguori wrote:
>>
>> For reasons that I do not fully understand, bdrv_aio_read() does
>> not return immediately, but instead it calls scsi_read_data()
>> recursively.
>
> This bothers me.  bdrv_aio_read() should never immediately invoke the
> callback to prevent exactly this sort of problem.  Perhaps this was a
> bug that has since been fixed?  Is this still reproducible?

qcow2 metadata is synchronous, and if the disk is empty, there will be
no data I/O, so bdrv_aio_read() will never be invoked.

Maybe we should fix this in qcow2 (and the other block formats) by
scheduling a BH.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.