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Commit 6b065462 authored by Dennis Zhou (Facebook)'s avatar Dennis Zhou (Facebook) Committed by Jens Axboe
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Revert "blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()" 



This reverts commit 4c699480.

Destroying blkgs is tricky because of the nature of the relationship. A
blkg should go away when either a blkcg or a request_queue goes away.
However, blkg's pin the blkcg to ensure they remain valid. To break this
cycle, when a blkcg is offlined, blkgs put back their css ref. This
eventually lets css_free() get called which frees the blkcg.

The above commit (4c699480) breaks this order of events by trying to
destroy blkgs in css_free(). As the blkgs still hold references to the
blkcg, css_free() is never called.

The race between blkcg_bio_issue_check() and cgroup_rmdir() will be
addressed in the following patch by delaying destruction of a blkg until
all writeback associated with the blkcg has been finished.

Fixes: 4c699480 ("blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()")
Reviewed-by: default avatarJosef Bacik <josef@toxicpanda.com>
Signed-off-by: default avatarDennis Zhou <dennisszhou@gmail.com>
Cc: Jiufei Xue <jiufei.xue@linux.alibaba.com>
Cc: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 52bd456a

block/blk-cgroup.c

+16 −62
Original line number Diff line number Diff line
@@ -310,28 +310,11 @@ struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg, 
	}

}



static void blkg_pd_offline(struct blkcg_gq *blkg)

{

	int i;



	lockdep_assert_held(blkg->q->queue_lock);

	lockdep_assert_held(&blkg->blkcg->lock);



	for (i = 0; i < BLKCG_MAX_POLS; i++) {

		struct blkcg_policy *pol = blkcg_policy[i];



		if (blkg->pd[i] && !blkg->pd[i]->offline &&

		    pol->pd_offline_fn) {

			pol->pd_offline_fn(blkg->pd[i]);

			blkg->pd[i]->offline = true;

		}

	}

}



static void blkg_destroy(struct blkcg_gq *blkg)

{

	struct blkcg *blkcg = blkg->blkcg;

	struct blkcg_gq *parent = blkg->parent;

	int i;



	lockdep_assert_held(blkg->q->queue_lock);

	lockdep_assert_held(&blkcg->lock);
@@ -340,6 +323,13 @@ static void blkg_destroy(struct blkcg_gq *blkg) 
	WARN_ON_ONCE(list_empty(&blkg->q_node));

	WARN_ON_ONCE(hlist_unhashed(&blkg->blkcg_node));



	for (i = 0; i < BLKCG_MAX_POLS; i++) {

		struct blkcg_policy *pol = blkcg_policy[i];



		if (blkg->pd[i] && pol->pd_offline_fn)

			pol->pd_offline_fn(blkg->pd[i]);

	}



	if (parent) {

		blkg_rwstat_add_aux(&parent->stat_bytes, &blkg->stat_bytes);

		blkg_rwstat_add_aux(&parent->stat_ios, &blkg->stat_ios);
@@ -382,7 +372,6 @@ static void blkg_destroy_all(struct request_queue *q) 
		struct blkcg *blkcg = blkg->blkcg;



		spin_lock(&blkcg->lock);

		blkg_pd_offline(blkg);

		blkg_destroy(blkg);

		spin_unlock(&blkcg->lock);

	}
@@ -1058,54 +1047,21 @@ static struct cftype blkcg_legacy_files[] = { 
 * @css: css of interest

 *

 * This function is called when @css is about to go away and responsible

 * for offlining all blkgs pd and killing all wbs associated with @css.

 * blkgs pd offline should be done while holding both q and blkcg locks.

 * As blkcg lock is nested inside q lock, this function performs reverse

 * double lock dancing.

 * for shooting down all blkgs associated with @css.  blkgs should be

 * removed while holding both q and blkcg locks.  As blkcg lock is nested

 * inside q lock, this function performs reverse double lock dancing.

 *

 * This is the blkcg counterpart of ioc_release_fn().

 */

static void blkcg_css_offline(struct cgroup_subsys_state *css)

{

	struct blkcg *blkcg = css_to_blkcg(css);

	struct blkcg_gq *blkg;



	spin_lock_irq(&blkcg->lock);



	hlist_for_each_entry(blkg, &blkcg->blkg_list, blkcg_node) {

		struct request_queue *q = blkg->q;



		if (spin_trylock(q->queue_lock)) {

			blkg_pd_offline(blkg);

			spin_unlock(q->queue_lock);

		} else {

			spin_unlock_irq(&blkcg->lock);

			cpu_relax();

			spin_lock_irq(&blkcg->lock);

		}

	}



	spin_unlock_irq(&blkcg->lock);



	wb_blkcg_offline(blkcg);

}



/**

 * blkcg_destroy_all_blkgs - destroy all blkgs associated with a blkcg

 * @blkcg: blkcg of interest

 *

 * This function is called when blkcg css is about to free and responsible for

 * destroying all blkgs associated with @blkcg.

 * blkgs should be removed while holding both q and blkcg locks. As blkcg lock

 * is nested inside q lock, this function performs reverse double lock dancing.

 */

static void blkcg_destroy_all_blkgs(struct blkcg *blkcg)

{

	spin_lock_irq(&blkcg->lock);

	while (!hlist_empty(&blkcg->blkg_list)) {

		struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first,

						    struct blkcg_gq,

						    blkcg_node);

						struct blkcg_gq, blkcg_node);

		struct request_queue *q = blkg->q;



		if (spin_trylock(q->queue_lock)) {
@@ -1117,7 +1073,10 @@ static void blkcg_destroy_all_blkgs(struct blkcg *blkcg) 
			spin_lock_irq(&blkcg->lock);

		}

	}



	spin_unlock_irq(&blkcg->lock);



	wb_blkcg_offline(blkcg);

}



static void blkcg_css_free(struct cgroup_subsys_state *css)
@@ -1125,8 +1084,6 @@ static void blkcg_css_free(struct cgroup_subsys_state *css) 
	struct blkcg *blkcg = css_to_blkcg(css);

	int i;



	blkcg_destroy_all_blkgs(blkcg);



	mutex_lock(&blkcg_pol_mutex);



	list_del(&blkcg->all_blkcgs_node);
@@ -1480,11 +1437,8 @@ void blkcg_deactivate_policy(struct request_queue *q, 


	list_for_each_entry(blkg, &q->blkg_list, q_node) {

		if (blkg->pd[pol->plid]) {

			if (!blkg->pd[pol->plid]->offline &&

			    pol->pd_offline_fn) {

			if (pol->pd_offline_fn)

				pol->pd_offline_fn(blkg->pd[pol->plid]);

				blkg->pd[pol->plid]->offline = true;

			}

			pol->pd_free_fn(blkg->pd[pol->plid]);

			blkg->pd[pol->plid] = NULL;

		}

include/linux/blk-cgroup.h

+0 −1
Original line number Diff line number Diff line
@@ -89,7 +89,6 @@ struct blkg_policy_data { 
	/* the blkg and policy id this per-policy data belongs to */

	struct blkcg_gq			*blkg;

	int				plid;

	bool				offline;

};



/*