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 | 78 ++++++++++++++++++++++++++++++++++++----------
include/linux/blk-cgroup.h | 1 +
2 files changed, 63 insertions(+), 16 deletions(-)
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index c2033a2..92112f4 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -307,11 +307,28 @@ 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]->offlined &&
+ pol->pd_offline_fn) {
+ pol->pd_offline_fn(blkg->pd[i]);
+ blkg->pd[i]->offlined = 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 +337,6 @@ 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 +379,7 @@ 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);
}
@@ -995,25 +1006,25 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
* @css: css of interest
*
* This function is called when @css is about to go away and responsible
- * 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.
+ * 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.
*
* 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);
- 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);
@@ -1027,11 +1038,43 @@ static void blkcg_css_offline(struct cgroup_subsys_state *css)
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 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);
+}
+
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);
@@ -1371,8 +1414,11 @@ 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[pol->plid]->offlined &&
+ pol->pd_offline_fn) {
pol->pd_offline_fn(blkg->pd[pol->plid]);
+ blkg->pd[pol->plid]->offlined = 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..dccd102 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -88,6 +88,7 @@ struct blkg_policy_data {
/* the blkg and policy id this per-policy data belongs to */
struct blkcg_gq *blkg;
int plid;
+ bool offlined;
};
/*
--
1.8.3.1
--
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