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

[Halil Pasic <pasic@linux.ibm.com>]

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?

* 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.
---
 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);
-- 
2.17.1