Re: panic with CPU hotplug + blk-mq + scsi-mq

[Jens Axboe]

On 04/17/2015 10:23 PM, Ming Lei wrote:
> Hi Dongsu,
>
> On Fri, Apr 17, 2015 at 5:41 AM, Dongsu Park
> <dongsu.park@xxxxxxxxxxxxxxxx> wrote:
> > Hi,
> >
> > there's a critical bug regarding CPU hotplug, blk-mq, and scsi-mq.
> > Every time when a CPU is offlined, some arbitrary range of kernel memory
> > seems to get corrupted. Then after a while, kernel panics at random places
> > when block IOs are issued. (for example, see the call traces below)
>
> Thanks for the report.
>
> > This bug can be easily reproducible with a Qemu VM running with virtio-scsi,
> > when its guest kernel is 3.19-rc1 or higher, and when scsi-mq is loaded
> > with blk-mq enabled. And yes, 4.0 release is still affected, as well as
> > Jens' for-4.1/core. How to reproduce:
> >
> >   # echo 0 > /sys/devices/system/cpu/cpu1/online
> >   (and issue some block IOs, that's it.)
> >
> > Bisecting between 3.18 and 3.19-rc1, it looks like this bug had been hidden
> > until commit ccbedf117f01 ("virtio_scsi: support multi hw queue of blk-mq"),
> > which started to allow virtio-scsi to map virtqueues to hardware queues of
> > blk-mq. Reverting that commit makes the bug go away. However, I suppose
> > reverting it could not be a correct solution.
>
> I agree, and that patch only enables multiple hw queues.
>
> > More precisely, every time a CPU hotplug event gets triggered,
> > a call graph is like the following:
> >
> >    blk_mq_queue_reinit_notify()
> >    -> blk_mq_queue_reinit()
> >     -> blk_mq_map_swqueue()
> >      -> blk_mq_free_rq_map()
> >       -> scsi_exit_request()
> >
> >  From that point, as soon as any address in the request gets modified, an
> > arbitrary range of memory gets corrupted. My first guess was that probably
> > the exit routine could try to deallocate tags->rqs[] where invalid
> > addresses are stored. But actually it looks like it's not the case,
> > and cmd->sense_buffer looks also valid.
> > It's not obvious to me, exactly what could go wrong.
> >
> > Does anyone have an idea?
>
> As far as I can see, at least two problems exist:
> - race between timeout and CPU hotplug
> - in case of shared tags, during CPU online handling, about setting
> and checking hctx->tags
>
> So could you please test the attached two patches to see if they fix your issue?
>
> I run them in my VM, and looks opps does disappear.

Hard to comment on your patches directly when they are attached. Both 
look good to me. I'd perhaps change the ->tags check in #1 to use 
blk_mq_hw_queue_mapped() instead of checking directly. Might even be 
worth considering changing the normal iterator to skip unmapped queues, 
but that can be left for a later change.

--
Jens Axboe

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