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

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 24.07.19 10:34, Cornelia Huck wrote:
> 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?

I think we can you did via a fast path. Halil, can you push to the s390 tree?