Hello Tejun,
Could you please help review this version?
Thanks,
Joseph
On 18/3/6 11:53, Joseph Qi wrote:
> 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 commit ae1188963611 ("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: ae1188963611 ("blkcg: consolidate blkg creation in blkcg_bio_issue_check()")
> Reported-by: Jiufei Xue <jiufei.xue@xxxxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx #4.3+
> Signed-off-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
> ---
> block/blk-cgroup.c | 57 +++++++++++++++++++++++++++++++++++-----------
> include/linux/blk-cgroup.h | 2 ++
> 2 files changed, 46 insertions(+), 13 deletions(-)
>
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index c2033a2..450cefd 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -307,11 +307,27 @@ 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_offline[i] && pol->pd_offline_fn) {
> + pol->pd_offline_fn(blkg->pd[i]);
> + blkg->pd_offline[i] = 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);
> @@ -320,13 +336,6 @@ 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);
> @@ -369,6 +378,7 @@ 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);
> }
> @@ -1004,16 +1014,15 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
> 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);
>
> - while (!hlist_empty(&blkcg->blkg_list)) {
> - struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first,
> - struct blkcg_gq, blkcg_node);
> + hlist_for_each_entry(blkg, &blkcg->blkg_list, blkcg_node) {
> struct request_queue *q = blkg->q;
>
> if (spin_trylock(q->queue_lock)) {
> - blkg_destroy(blkg);
> + blkg_pd_offline(blkg);
> spin_unlock(q->queue_lock);
> } else {
> spin_unlock_irq(&blkcg->lock);
> @@ -1032,6 +1041,26 @@ static void blkcg_css_free(struct cgroup_subsys_state *css)
> struct blkcg *blkcg = css_to_blkcg(css);
> int i;
>
> + 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 request_queue *q = blkg->q;
> +
> + if (spin_trylock(q->queue_lock)) {
> + blkg_destroy(blkg);
> + spin_unlock(q->queue_lock);
> + } else {
> + spin_unlock_irq(&blkcg->lock);
> + cpu_relax();
> + spin_lock_irq(&blkcg->lock);
> + }
> + }
> +
> + spin_unlock_irq(&blkcg->lock);
> +
> mutex_lock(&blkcg_pol_mutex);
>
> list_del(&blkcg->all_blkcgs_node);
> @@ -1371,8 +1400,10 @@ void blkcg_deactivate_policy(struct request_queue *q,
> spin_lock(&blkg->blkcg->lock);
>
> if (blkg->pd[pol->plid]) {
> - if (pol->pd_offline_fn)
> + if (!blkg->pd_offline[pol->plid] && pol->pd_offline_fn) {
> pol->pd_offline_fn(blkg->pd[pol->plid]);
> + blkg->pd_offline[pol->plid] = true;
> + }
> pol->pd_free_fn(blkg->pd[pol->plid]);
> blkg->pd[pol->plid] = NULL;
> }
> diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
> index 69bea82..ad5e2cb 100644
> --- a/include/linux/blk-cgroup.h
> +++ b/include/linux/blk-cgroup.h
> @@ -133,6 +133,8 @@ struct blkcg_gq {
> struct blkg_rwstat stat_ios;
>
> struct blkg_policy_data *pd[BLKCG_MAX_POLS];
> + /* is the corresponding policy data offline? */
> + bool pd_offline[BLKCG_MAX_POLS];
>
> struct rcu_head rcu_head;
> };
>
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