dm cache policy mq: protect residency method with existing mutex

It is safe to use a mutex in mq_residency() at this point since it is
only called from ioctl context.  But future-proof mq_residency() by
using might_sleep() to catch new contexts that cannot sleep.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/drivers/md/dm-cache-policy-mq.c b/drivers/md/dm-cache-policy-mq.c
index 4296155090b2..db490f74c7f8 100644
--- a/drivers/md/dm-cache-policy-mq.c
+++ b/drivers/md/dm-cache-policy-mq.c
@@ -1001,10 +1001,14 @@ static void mq_force_mapping(struct dm_cache_policy *p,
 
 static dm_cblock_t mq_residency(struct dm_cache_policy *p)
 {
+        dm_cblock_t r;
         struct mq_policy *mq = to_mq_policy(p);
 
-        /* FIXME: lock mutex, not sure we can block here */
-        return to_cblock(mq->nr_cblocks_allocated);
+        mutex_lock(&mq->lock);
+        r = to_cblock(mq->nr_cblocks_allocated);
+        mutex_unlock(&mq->lock);
+
+        return r;
 }
 
 static void mq_tick(struct dm_cache_policy *p)