Re: [Qemu-devel] [PATCH] vl.c/exit: pause cpus before closing block devices

[John Snow <jsnow@redhat.com>]

On 07/17/2017 06:26 AM, Dr. David Alan Gilbert wrote:
> * Stefan Hajnoczi (stefanha@gmail.com) wrote:
>> On Thu, Jul 13, 2017 at 08:01:16PM +0100, Dr. David Alan Gilbert (git) wrote:
>>> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>>>
>>> There's a rare exit seg if the guest is accessing
>>> IO during exit.
>>> It's always hitting the atomic_inc(&bs->in_flight) with a NULL
>>> bs. This was added recently in 99723548  but I don't see it
>>> as the cause.
>>>
>>> Flip vl.c around so we pause the cpus before closing the block devices,
>>> that way we shouldn't have anything trying to access them when
>>> they're gone.
>>>
>>> This was originally Red Hat bz https://bugzilla.redhat.com/show_bug.cgi?id=1451015
>>>
>>> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>>> Reported-by: Cong Li <coli@redhat.com>
>>>
>>> --
>>> This is a very rare race, I'll leave it running in a loop to see if
>>> we hit anything else and to check this really fixes it.
>>>
>>> I do worry if there are other cases that can trigger this - e.g.
>>> hot-unplug or ejecting a CD.
>>>
>>> ---
>>>  vl.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> 
> Thanks;  and the test I left running seems solid - ~12k runs
> over the weekend with no seg.
> 
> Dave
> 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 

the root cause of this bug is related to this as well:
https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg02945.html

>From commit 99723548 we started assuming (incorrectly?) that blk_
functions always WILL have an attached BDS, but this is not always true,
for instance, flushing the cache from an empty CDROM.

Paolo, can we move the flight counter increment outside of the
block-backend layer, is that safe?

--js