blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()
We've triggered a WARNING in blk_throtl_bio() when throttling writeback io, which complains blkg->refcnt is already 0 when calling blkg_get(), and then kernel crashes with invalid page request. After investigating this issue, we've found it is caused by a race between blkcg_bio_issue_check() and cgroup_rmdir(), which is described below: writeback kworker cgroup_rmdir cgroup_destroy_locked kill_css css_killed_ref_fn css_killed_work_fn offline_css blkcg_css_offline blkcg_bio_issue_check rcu_read_lock blkg_lookup spin_trylock(q->queue_lock) blkg_destroy spin_unlock(q->queue_lock) blk_throtl_bio spin_lock_irq(q->queue_lock) ... spin_unlock_irq(q->queue_lock) rcu_read_unlock Since rcu can only prevent blkg from releasing when it is being used, the blkg->refcnt can be decreased to 0 during blkg_destroy() and schedule blkg release. Then trying to blkg_get() in blk_throtl_bio() will complains the WARNING. And then the corresponding blkg_put() will schedule blkg release again, which result in double free. This race is introduced by commitae11889636
("blkcg: consolidate blkg creation in blkcg_bio_issue_check()"). Before this commit, it will lookup first and then try to lookup/create again with queue_lock. Since revive this logic is a bit drastic, so fix it by only offlining pd during blkcg_css_offline(), and move the rest destruction (especially blkg_put()) into blkcg_css_free(), which should be the right way as discussed. Fixes:ae11889636
("blkcg: consolidate blkg creation in blkcg_bio_issue_check()") Reported-by: Jiufei Xue <jiufei.xue@linux.alibaba.com> Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com> Acked-by: Tejun Heo <tj@kernel.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
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@ -307,11 +307,28 @@ struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
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}
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}
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static void blkg_pd_offline(struct blkcg_gq *blkg)
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{
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int i;
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lockdep_assert_held(blkg->q->queue_lock);
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lockdep_assert_held(&blkg->blkcg->lock);
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for (i = 0; i < BLKCG_MAX_POLS; i++) {
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struct blkcg_policy *pol = blkcg_policy[i];
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if (blkg->pd[i] && !blkg->pd[i]->offline &&
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pol->pd_offline_fn) {
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pol->pd_offline_fn(blkg->pd[i]);
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blkg->pd[i]->offline = true;
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}
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}
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}
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static void blkg_destroy(struct blkcg_gq *blkg)
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{
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struct blkcg *blkcg = blkg->blkcg;
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struct blkcg_gq *parent = blkg->parent;
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int i;
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lockdep_assert_held(blkg->q->queue_lock);
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lockdep_assert_held(&blkcg->lock);
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@ -320,13 +337,6 @@ static void blkg_destroy(struct blkcg_gq *blkg)
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WARN_ON_ONCE(list_empty(&blkg->q_node));
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WARN_ON_ONCE(hlist_unhashed(&blkg->blkcg_node));
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for (i = 0; i < BLKCG_MAX_POLS; i++) {
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struct blkcg_policy *pol = blkcg_policy[i];
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if (blkg->pd[i] && pol->pd_offline_fn)
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pol->pd_offline_fn(blkg->pd[i]);
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}
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if (parent) {
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blkg_rwstat_add_aux(&parent->stat_bytes, &blkg->stat_bytes);
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blkg_rwstat_add_aux(&parent->stat_ios, &blkg->stat_ios);
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@ -369,6 +379,7 @@ static void blkg_destroy_all(struct request_queue *q)
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struct blkcg *blkcg = blkg->blkcg;
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spin_lock(&blkcg->lock);
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blkg_pd_offline(blkg);
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blkg_destroy(blkg);
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spin_unlock(&blkcg->lock);
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}
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@ -995,25 +1006,25 @@ static struct cftype blkcg_legacy_files[] = {
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* @css: css of interest
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*
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* This function is called when @css is about to go away and responsible
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* for shooting down all blkgs associated with @css. blkgs should be
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* removed while holding both q and blkcg locks. As blkcg lock is nested
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* inside q lock, this function performs reverse double lock dancing.
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* for offlining all blkgs pd and killing all wbs associated with @css.
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* blkgs pd offline should be done while holding both q and blkcg locks.
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* As blkcg lock is nested inside q lock, this function performs reverse
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* double lock dancing.
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*
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* This is the blkcg counterpart of ioc_release_fn().
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*/
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static void blkcg_css_offline(struct cgroup_subsys_state *css)
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{
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struct blkcg *blkcg = css_to_blkcg(css);
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struct blkcg_gq *blkg;
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spin_lock_irq(&blkcg->lock);
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while (!hlist_empty(&blkcg->blkg_list)) {
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struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first,
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struct blkcg_gq, blkcg_node);
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hlist_for_each_entry(blkg, &blkcg->blkg_list, blkcg_node) {
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struct request_queue *q = blkg->q;
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if (spin_trylock(q->queue_lock)) {
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blkg_destroy(blkg);
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blkg_pd_offline(blkg);
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spin_unlock(q->queue_lock);
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} else {
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spin_unlock_irq(&blkcg->lock);
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@ -1027,11 +1038,43 @@ static void blkcg_css_offline(struct cgroup_subsys_state *css)
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wb_blkcg_offline(blkcg);
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}
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/**
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* blkcg_destroy_all_blkgs - destroy all blkgs associated with a blkcg
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* @blkcg: blkcg of interest
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*
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* This function is called when blkcg css is about to free and responsible for
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* destroying all blkgs associated with @blkcg.
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* blkgs should be removed while holding both q and blkcg locks. As blkcg lock
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* is nested inside q lock, this function performs reverse double lock dancing.
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*/
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static void blkcg_destroy_all_blkgs(struct blkcg *blkcg)
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{
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spin_lock_irq(&blkcg->lock);
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while (!hlist_empty(&blkcg->blkg_list)) {
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struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first,
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struct blkcg_gq,
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blkcg_node);
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struct request_queue *q = blkg->q;
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if (spin_trylock(q->queue_lock)) {
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blkg_destroy(blkg);
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spin_unlock(q->queue_lock);
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} else {
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spin_unlock_irq(&blkcg->lock);
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cpu_relax();
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spin_lock_irq(&blkcg->lock);
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}
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}
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spin_unlock_irq(&blkcg->lock);
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}
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static void blkcg_css_free(struct cgroup_subsys_state *css)
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{
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struct blkcg *blkcg = css_to_blkcg(css);
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int i;
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blkcg_destroy_all_blkgs(blkcg);
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mutex_lock(&blkcg_pol_mutex);
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list_del(&blkcg->all_blkcgs_node);
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@ -1371,8 +1414,11 @@ void blkcg_deactivate_policy(struct request_queue *q,
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spin_lock(&blkg->blkcg->lock);
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if (blkg->pd[pol->plid]) {
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if (pol->pd_offline_fn)
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if (!blkg->pd[pol->plid]->offline &&
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pol->pd_offline_fn) {
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pol->pd_offline_fn(blkg->pd[pol->plid]);
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blkg->pd[pol->plid]->offline = true;
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}
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pol->pd_free_fn(blkg->pd[pol->plid]);
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blkg->pd[pol->plid] = NULL;
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}
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@ -88,6 +88,7 @@ struct blkg_policy_data {
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/* the blkg and policy id this per-policy data belongs to */
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struct blkcg_gq *blkg;
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int plid;
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bool offline;
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};
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/*
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