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Commit 9f13ef67 authored by Tejun Heo's avatar Tejun Heo Committed by Jens Axboe
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blkcg: use double locking instead of RCU for blkg synchronization 



blkgs are chained from both blkcgs and request_queues and thus
subjected to two locks - blkcg->lock and q->queue_lock.  As both blkcg
and q can go away anytime, locking during removal is tricky.  It's
currently solved by wrapping removal inside RCU, which makes the
synchronization complex.  There are three locks to worry about - the
outer RCU, q lock and blkcg lock, and it leads to nasty subtle
complications like conditional synchronize_rcu() on queue exit paths.

For all other paths, blkcg lock is naturally nested inside q lock and
the only exception is blkcg removal path, which is a very cold path
and can be implemented as clumsy but conceptually-simple reverse
double lock dancing.

This patch updates blkg removal path such that blkgs are removed while
holding both q and blkcg locks, which is trivial for request queue
exit path - blkg_destroy_all().  The blkcg removal path,
blkiocg_pre_destroy(), implements reverse double lock dancing
essentially identical to ioc_release_fn().

This simplifies blkg locking - no half-dead blkgs to worry about.  Now
unnecessary RCU annotations will be removed by the next patch.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent e8989fae

block/blk-cgroup.c

+51 −85
Original line number Diff line number Diff line
@@ -620,32 +620,6 @@ struct blkio_group *blkg_lookup_create(struct blkio_cgroup *blkcg, 
}

EXPORT_SYMBOL_GPL(blkg_lookup_create);



static void __blkiocg_del_blkio_group(struct blkio_group *blkg)

{

	hlist_del_init_rcu(&blkg->blkcg_node);

}



/*

 * returns 0 if blkio_group was still on cgroup list. Otherwise returns 1

 * indicating that blk_group was unhashed by the time we got to it.

 */

int blkiocg_del_blkio_group(struct blkio_group *blkg)

{

	struct blkio_cgroup *blkcg = blkg->blkcg;

	unsigned long flags;

	int ret = 1;



	spin_lock_irqsave(&blkcg->lock, flags);

	if (!hlist_unhashed(&blkg->blkcg_node)) {

		__blkiocg_del_blkio_group(blkg);

		ret = 0;

	}

	spin_unlock_irqrestore(&blkcg->lock, flags);



	return ret;

}

EXPORT_SYMBOL_GPL(blkiocg_del_blkio_group);



/* called under rcu_read_lock(). */

struct blkio_group *blkg_lookup(struct blkio_cgroup *blkcg,

				struct request_queue *q)
@@ -663,12 +637,16 @@ EXPORT_SYMBOL_GPL(blkg_lookup); 
static void blkg_destroy(struct blkio_group *blkg)

{

	struct request_queue *q = blkg->q;

	struct blkio_cgroup *blkcg = blkg->blkcg;



	lockdep_assert_held(q->queue_lock);

	lockdep_assert_held(&blkcg->lock);



	/* Something wrong if we are trying to remove same group twice */

	WARN_ON_ONCE(list_empty(&blkg->q_node));

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

	list_del_init(&blkg->q_node);

	hlist_del_init_rcu(&blkg->blkcg_node);



	WARN_ON_ONCE(q->nr_blkgs <= 0);

	q->nr_blkgs--;
@@ -713,45 +691,33 @@ void update_root_blkg_pd(struct request_queue *q, enum blkio_policy_id plid) 
}

EXPORT_SYMBOL_GPL(update_root_blkg_pd);



/**

 * blkg_destroy_all - destroy all blkgs associated with a request_queue

 * @q: request_queue of interest

 * @destroy_root: whether to destroy root blkg or not

 *

 * Destroy blkgs associated with @q.  If @destroy_root is %true, all are

 * destroyed; otherwise, root blkg is left alone.

 */

void blkg_destroy_all(struct request_queue *q, bool destroy_root)

{

	struct blkio_group *blkg, *n;



	while (true) {

		bool done = true;



	spin_lock_irq(q->queue_lock);



	list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {

		struct blkio_cgroup *blkcg = blkg->blkcg;



		/* skip root? */

		if (!destroy_root && blkg->blkcg == &blkio_root_cgroup)

			continue;



			/*

			 * If cgroup removal path got to blk_group first

			 * and removed it from cgroup list, then it will

			 * take care of destroying cfqg also.

			 */

			if (!blkiocg_del_blkio_group(blkg))

		spin_lock(&blkcg->lock);

		blkg_destroy(blkg);

			else

				done = false;

		spin_unlock(&blkcg->lock);

	}



	spin_unlock_irq(q->queue_lock);



		/*

		 * Group list may not be empty if we raced cgroup removal

		 * and lost.  cgroup removal is guaranteed to make forward

		 * progress and retrying after a while is enough.  This

		 * ugliness is scheduled to be removed after locking

		 * update.

		 */

		if (done)

			break;



		msleep(10);	/* just some random duration I like */

	}

}

EXPORT_SYMBOL_GPL(blkg_destroy_all);
@@ -1600,45 +1566,45 @@ static int blkiocg_populate(struct cgroup_subsys *subsys, struct cgroup *cgroup) 
				ARRAY_SIZE(blkio_files));

}



/**

 * blkiocg_pre_destroy - cgroup pre_destroy callback

 * @subsys: cgroup subsys

 * @cgroup: cgroup of interest

 *

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

 * for shooting down all blkgs associated with @cgroup.  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 int blkiocg_pre_destroy(struct cgroup_subsys *subsys,

			       struct cgroup *cgroup)

{

	struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgroup);

	unsigned long flags;

	struct blkio_group *blkg;

	struct request_queue *q;



	rcu_read_lock();

	spin_lock_irq(&blkcg->lock);



	do {

		spin_lock_irqsave(&blkcg->lock, flags);



		if (hlist_empty(&blkcg->blkg_list)) {

			spin_unlock_irqrestore(&blkcg->lock, flags);

			break;

		}



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

					blkcg_node);

		q = rcu_dereference(blkg->q);

		__blkiocg_del_blkio_group(blkg);



		spin_unlock_irqrestore(&blkcg->lock, flags);

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

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

						struct blkio_group, blkcg_node);

		struct request_queue *q = rcu_dereference(blkg->q);



		/*

		 * This blkio_group is being unlinked as associated cgroup is

		 * going away. Let all the IO controlling policies know about

		 * this event.

		 */

		spin_lock(&blkio_list_lock);

		spin_lock_irqsave(q->queue_lock, flags);

		if (spin_trylock(q->queue_lock)) {

			blkg_destroy(blkg);

		spin_unlock_irqrestore(q->queue_lock, flags);

		spin_unlock(&blkio_list_lock);

	} while (1);



			spin_unlock(q->queue_lock);

		} else {

			spin_unlock_irq(&blkcg->lock);

			rcu_read_unlock();

			cpu_relax();

			rcu_read_lock();

			spin_lock(&blkcg->lock);

		}

	}



	spin_unlock_irq(&blkcg->lock);

	rcu_read_unlock();

	return 0;

}

block/blk-cgroup.h

+0 −4
Original line number Diff line number Diff line
@@ -380,7 +380,6 @@ static inline void blkiocg_set_start_empty_time(struct blkio_group *blkg, 
extern struct blkio_cgroup blkio_root_cgroup;

extern struct blkio_cgroup *cgroup_to_blkio_cgroup(struct cgroup *cgroup);

extern struct blkio_cgroup *task_blkio_cgroup(struct task_struct *tsk);

extern int blkiocg_del_blkio_group(struct blkio_group *blkg);

extern struct blkio_group *blkg_lookup(struct blkio_cgroup *blkcg,

				       struct request_queue *q);

struct blkio_group *blkg_lookup_create(struct blkio_cgroup *blkcg,
@@ -416,9 +415,6 @@ cgroup_to_blkio_cgroup(struct cgroup *cgroup) { return NULL; } 
static inline struct blkio_cgroup *

task_blkio_cgroup(struct task_struct *tsk) { return NULL; }



static inline int

blkiocg_del_blkio_group(struct blkio_group *blkg) { return 0; }



static inline struct blkio_group *blkg_lookup(struct blkio_cgroup *blkcg,

					      void *key) { return NULL; }

static inline void blkiocg_update_timeslice_used(struct blkio_group *blkg,

block/cfq.h

+0 −10
Original line number Diff line number Diff line
@@ -79,11 +79,6 @@ static inline void cfq_blkiocg_update_completion_stats(struct blkio_group *blkg, 
					direction, sync);

}



static inline int cfq_blkiocg_del_blkio_group(struct blkio_group *blkg)

{

	return blkiocg_del_blkio_group(blkg);

}



#else /* CFQ_GROUP_IOSCHED */

static inline void cfq_blkiocg_update_io_add_stats(struct blkio_group *blkg,

			struct blkio_policy_type *pol,
@@ -119,10 +114,5 @@ static inline void cfq_blkiocg_update_completion_stats(struct blkio_group *blkg, 
			struct blkio_policy_type *pol, uint64_t start_time,

			uint64_t io_start_time, bool direction, bool sync) { }



static inline int cfq_blkiocg_del_blkio_group(struct blkio_group *blkg)

{

	return 0;

}



#endif /* CFQ_GROUP_IOSCHED */

#endif