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Commit 6ab21879 authored by Dennis Zhou's avatar Dennis Zhou Committed by Jens Axboe
Browse files

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: default avatarDennis Zhou <dennis@kernel.org>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 5816a093

block/bio.c

+4 −0
Original line number Diff line number Diff line
@@ -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 */

include/linux/blk-cgroup.h

+16 −5
Original line number Diff line number Diff line
@@ -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))

	struct blkcg_gq *ret_blkg = NULL;



	WARN_ON_ONCE(!rcu_read_lock_held());



	while (blkg) {

		if (blkg_tryget(blkg)) {

			ret_blkg = blkg;

			break;

		}

		blkg = blkg->parent;

	}



	return blkg;

	return ret_blkg;

}



/**