Re: [PATCH 1/1] virtio/s390: fix race on airq_areas[]

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 24 Jul 2019 08:44:19 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> On 24.07.19 00:58, Halil Pasic wrote:
> > The access to airq_areas was racy ever since the adapter interrupts got
> > introduced to virtio-ccw, but since commit 39c7dcb15892 ("virtio/s390:
> > make airq summary indicators DMA") this became an issue in practice as
> > well. Namely before that commit the airq_info that got overwritten was
> > still functional. After that commit however the two infos share a
> > summary_indicator, which aggravates the situation. Which means
> > auto-online mechanism occasionally hangs the boot with virtio_blk.
> > 
> > Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> > Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
> > Fixes: 96b14536d935 ("virtio-ccw: virtio-ccw adapter interrupt support.")
> > ---
> > * We need definitely this fixed for 5.3. For older stable kernels it is
> > to be discussed. @Connie what do you think: do we need a cc stable?  
> 
> Unless you can prove that the problem could never happen on old version
> we absolutely do need cc stable. 

Yes, this needs to be cc:stable.

> 
> > 
> > * I have a variant that does not need the extra mutex but uses cmpxchg().
> > Decided to post this one because that one is more complex. But if there
> > is interest we can have a look at it as well.  
> 
> This is slow path (startup) and never called in hot path. Correct? Mutex should be
> fine.

Yes, this is ultimately called through the ->probe functions of virtio
drivers.

> > ---
> >  drivers/s390/virtio/virtio_ccw.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c
> > index 1a55e5942d36..d97742662755 100644
> > --- a/drivers/s390/virtio/virtio_ccw.c
> > +++ b/drivers/s390/virtio/virtio_ccw.c
> > @@ -145,6 +145,8 @@ struct airq_info {
> >  	struct airq_iv *aiv;
> >  };
> >  static struct airq_info *airq_areas[MAX_AIRQ_AREAS];
> > +DEFINE_MUTEX(airq_areas_lock);
> > +
> >  static u8 *summary_indicators;
> >  
> >  static inline u8 *get_summary_indicator(struct airq_info *info)
> > @@ -265,9 +267,11 @@ static unsigned long get_airq_indicator(struct virtqueue *vqs[], int nvqs,
> >  	unsigned long bit, flags;
> >  
> >  	for (i = 0; i < MAX_AIRQ_AREAS && !indicator_addr; i++) {
> > +		mutex_lock(&airq_areas_lock);
> >  		if (!airq_areas[i])
> >  			airq_areas[i] = new_airq_info(i);
> >  		info = airq_areas[i];
> > +		mutex_unlock(&airq_areas_lock);
> >  		if (!info)
> >  			return 0;
> >  		write_lock_irqsave(&info->lock, flags);
> >   
> 

Reviewed-by: Cornelia Huck <cohuck@redhat.com>

Should I pick this and send a pull request, or is it quicker to just
take this directly?