Commit dae590a6 authored by Waiman Long's avatar Waiman Long Committed by Jens Axboe
Browse files

blk-cgroup: Flush stats at blkgs destruction path 



As noted by Michal, the blkg_iostat_set's in the lockless list
hold reference to blkg's to protect against their removal. Those
blkg's hold reference to blkcg. When a cgroup is being destroyed,
cgroup_rstat_flush() is only called at css_release_work_fn() which is
called when the blkcg reference count reaches 0. This circular dependency
will prevent blkcg from being freed until some other events cause
cgroup_rstat_flush() to be called to flush out the pending blkcg stats.

To prevent this delayed blkcg removal, add a new cgroup_rstat_css_flush()
function to flush stats for a given css and cpu and call it at the blkgs
destruction path, blkcg_destroy_blkgs(), whenever there are still some
pending stats to be flushed. This will ensure that blkcg reference
count can reach 0 ASAP.

Signed-off-by: default avatarWaiman Long <longman@redhat.com>
Acked-by: default avatarTejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20221105005902.407297-4-longman@redhat.com


Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 3b8cc629

block/blk-cgroup.c

+14 −1
Original line number Diff line number Diff line
@@ -1084,10 +1084,12 @@ struct list_head *blkcg_get_cgwb_list(struct cgroup_subsys_state *css) 
 */

static void blkcg_destroy_blkgs(struct blkcg *blkcg)

{

	int cpu;



	might_sleep();



	css_get(&blkcg->css);

	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);
@@ -1110,6 +1112,17 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg) 
	}



	spin_unlock_irq(&blkcg->lock);



	/*

	 * Flush all the non-empty percpu lockless lists.

	 */

	for_each_possible_cpu(cpu) {

		struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);



		if (!llist_empty(lhead))

			cgroup_rstat_css_cpu_flush(&blkcg->css, cpu);

	}

	css_put(&blkcg->css);

}



/**

include/linux/cgroup.h

+1 −0
Original line number Diff line number Diff line
@@ -766,6 +766,7 @@ void cgroup_rstat_flush(struct cgroup *cgrp); 
void cgroup_rstat_flush_irqsafe(struct cgroup *cgrp);

void cgroup_rstat_flush_hold(struct cgroup *cgrp);

void cgroup_rstat_flush_release(void);

void cgroup_rstat_css_cpu_flush(struct cgroup_subsys_state *css, int cpu);



/*

 * Basic resource stats.

kernel/cgroup/rstat.c

+20 −0
Original line number Diff line number Diff line
@@ -281,6 +281,26 @@ void cgroup_rstat_flush_release(void) 
	spin_unlock_irq(&cgroup_rstat_lock);

}



/**

 * cgroup_rstat_css_cpu_flush - flush stats for the given css and cpu

 * @css: target css to be flush

 * @cpu: the cpu that holds the stats to be flush

 *

 * A lightweight rstat flush operation for a given css and cpu.

 * Only the cpu_lock is being held for mutual exclusion, the cgroup_rstat_lock

 * isn't used.

 */

void cgroup_rstat_css_cpu_flush(struct cgroup_subsys_state *css, int cpu)

{

	raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);



	raw_spin_lock_irq(cpu_lock);

	rcu_read_lock();

	css->ss->css_rstat_flush(css, cpu);

	rcu_read_unlock();

	raw_spin_unlock_irq(cpu_lock);

}



int cgroup_rstat_init(struct cgroup *cgrp)

{

	int cpu;