[PATCH 01/15] Revert "blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()"

Dennis Zhou
2018-08-31T01:53:42Z

From: "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>

This reverts commit 4c6994806f708559c2812b73501406e21ae5dcd0.

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 (4c6994806f70) 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: 4c6994806f70 ("blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()")
Signed-off-by: Dennis 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>
---
block/blk-cgroup.c | 78 ++++++++------------------------------
include/linux/blk-cgroup.h | 1 -
2 files changed, 16 insertions(+), 63 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 694595b29b8f..2998e4f095d1 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -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;
}
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index 34aec30e06c7..1615cdd4c797 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -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;
};

/*
--
2.17.1