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blkcg: clean up blkg_tryget_closest()

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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>
This commit is contained in:
Dennis Zhou 2018-12-19 16:43:21 -06:00 committed by Jens Axboe
parent 5816a0932b
commit 6ab2187992
2 changed files with 21 additions and 6 deletions
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4
block/bio.c
View File

@ -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 */

23
include/linux/blk-cgroup.h
View File

@ -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;
}
/**