blkcg: clean up blkg_tryget_closest()
The implementation of blkg_tryget_closest() wasn't super obvious and
became a point of suspicion when debugging [1]. So let's clean it up so
it's obviously not the problem.
Also add missing RCU read locking to bio_clone_blkg_association(), which
got exposed by adding the RCU read lock held check in
blkg_tryget_closest().
[1] https://lore.kernel.org/linux-block/a7e97e4b-0dd8-3a54-23b7-a0f27b17fde8@kernel.dk/
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/bio.c b/block/bio.c
index c288b90..9194d8a 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -2096,8 +2096,12 @@ EXPORT_SYMBOL_GPL(bio_associate_blkg);
*/
void bio_clone_blkg_association(struct bio *dst, struct bio *src)
{
+ rcu_read_lock();
+
if (src->bi_blkg)
__bio_associate_blkg(dst, src->bi_blkg);
+
+ rcu_read_unlock();
}
EXPORT_SYMBOL_GPL(bio_clone_blkg_association);
#endif /* CONFIG_BLK_CGROUP */
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index f025fd1..76c6131 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -499,22 +499,33 @@ static inline void blkg_get(struct blkcg_gq *blkg)
*/
static inline bool blkg_tryget(struct blkcg_gq *blkg)
{
- return percpu_ref_tryget(&blkg->refcnt);
+ return blkg && percpu_ref_tryget(&blkg->refcnt);
}
/**
* blkg_tryget_closest - try and get a blkg ref on the closet blkg
* @blkg: blkg to get
*
- * This walks up the blkg tree to find the closest non-dying blkg and returns
- * the blkg that it did association with as it may not be the passed in blkg.
+ * This needs to be called rcu protected. As the failure mode here is to walk
+ * up the blkg tree, this ensure that the blkg->parent pointers are always
+ * valid. This returns the blkg that it ended up taking a reference on or %NULL
+ * if no reference was taken.
*/
static inline struct blkcg_gq *blkg_tryget_closest(struct blkcg_gq *blkg)
{
- while (blkg && !percpu_ref_tryget(&blkg->refcnt))
- blkg = blkg->parent;
+ struct blkcg_gq *ret_blkg = NULL;
- return blkg;
+ WARN_ON_ONCE(!rcu_read_lock_held());
+
+ while (blkg) {
+ if (blkg_tryget(blkg)) {
+ ret_blkg = blkg;
+ break;
+ }
+ blkg = blkg->parent;
+ }
+
+ return ret_blkg;
}
/**