Re: [Qemu-devel] Re: [PATCH]: fix QEMU SCSI lock up

[Anthony Liguori]

Avi Kivity wrote:
> Avi Kivity wrote:
> > 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.
> >   
>
> FWIW, I was told this reproduces on kvm-77 (which has the latest qemu 
> scsi bits).

qemu_aio_wait() will run bottom halves when emulating synchronous IO.  I 
don't think this is exploitable practically speaking but it seems to me 
like a major flaw.  I think the proper fix is what you describe, 
modifying qcow2 to schedule a bottom half to read metadata.  Better yet, 
a full conversion to make the meta data reading/writing asynchronous.

Regards,

Anthony Liguori