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Date: Tue, 21 Feb 2012 17:46:55 -0800
From: Tejun Heo <tj@...nel.org>
To: axboe@...nel.dk, vgoyal@...hat.com
Cc: ctalbott@...gle.com, rni@...gle.com, linux-kernel@...r.kernel.org,
Tejun Heo <tj@...nel.org>
Subject: [PATCH 28/36] blkcg: use double locking instead of RCU for blkg synchronization
blkgs are chained from both blkcgs and request_queues and thus
subjected to two locks - blkcg->lock and q->queue_lock. As both blkcg
and q can go away anytime, locking during removal is tricky. It's
currently solved by wrapping removal inside RCU, which makes the
synchronization complex. There are three locks to worry about - the
outer RCU, q lock and blkcg lock, and it leads to nasty subtle
complications like conditional synchronize_rcu() on queue exit paths.
For all other paths, blkcg lock is naturally nested inside q lock and
the only exception is blkcg removal path, which is a very cold path
and can be implemented as clumsy but conceptually-simple reverse
double lock dancing.
This patch updates blkg removal path such that blkgs are removed while
holding both q and blkcg locks, which is trivial for request queue
exit path - blkg_destroy_all(). The blkcg removal path,
blkiocg_pre_destroy(), implements reverse double lock dancing
essentially identical to ioc_release_fn().
This simplifies blkg locking - no half-dead blkgs to worry about. Now
unnecessary RCU annotations will be removed by the next patch.
Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Vivek Goyal <vgoyal@...hat.com>
---
block/blk-cgroup.c | 136 +++++++++++++++++++--------------------------------
block/blk-cgroup.h | 4 --
block/cfq.h | 10 ----
3 files changed, 51 insertions(+), 99 deletions(-)
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index ce2dd15..aee71ef 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -620,32 +620,6 @@ out:
}
EXPORT_SYMBOL_GPL(blkg_lookup_create);
-static void __blkiocg_del_blkio_group(struct blkio_group *blkg)
-{
- hlist_del_init_rcu(&blkg->blkcg_node);
-}
-
-/*
- * returns 0 if blkio_group was still on cgroup list. Otherwise returns 1
- * indicating that blk_group was unhashed by the time we got to it.
- */
-int blkiocg_del_blkio_group(struct blkio_group *blkg)
-{
- struct blkio_cgroup *blkcg = blkg->blkcg;
- unsigned long flags;
- int ret = 1;
-
- spin_lock_irqsave(&blkcg->lock, flags);
- if (!hlist_unhashed(&blkg->blkcg_node)) {
- __blkiocg_del_blkio_group(blkg);
- ret = 0;
- }
- spin_unlock_irqrestore(&blkcg->lock, flags);
-
- return ret;
-}
-EXPORT_SYMBOL_GPL(blkiocg_del_blkio_group);
-
/* called under rcu_read_lock(). */
struct blkio_group *blkg_lookup(struct blkio_cgroup *blkcg,
struct request_queue *q)
@@ -663,12 +637,16 @@ EXPORT_SYMBOL_GPL(blkg_lookup);
static void blkg_destroy(struct blkio_group *blkg)
{
struct request_queue *q = blkg->q;
+ struct blkio_cgroup *blkcg = blkg->blkcg;
lockdep_assert_held(q->queue_lock);
+ lockdep_assert_held(&blkcg->lock);
/* Something wrong if we are trying to remove same group twice */
WARN_ON_ONCE(list_empty(&blkg->q_node));
+ WARN_ON_ONCE(hlist_unhashed(&blkg->blkcg_node));
list_del_init(&blkg->q_node);
+ hlist_del_init_rcu(&blkg->blkcg_node);
WARN_ON_ONCE(q->nr_blkgs <= 0);
q->nr_blkgs--;
@@ -712,47 +690,35 @@ static void update_root_blkg(struct request_queue *q, enum blkio_policy_id plid)
pol->ops.blkio_init_group_fn(blkg);
}
+/**
+ * blkg_destroy_all - destroy all blkgs associated with a request_queue
+ * @q: request_queue of interest
+ * @destroy_root: whether to destroy root blkg or not
+ *
+ * Destroy blkgs associated with @q. If @destroy_root is %true, all are
+ * destroyed; otherwise, root blkg is left alone.
+ */
void blkg_destroy_all(struct request_queue *q, bool destroy_root)
{
struct blkio_group *blkg, *n;
int i;
- while (true) {
- bool done = true;
-
- spin_lock_irq(q->queue_lock);
-
- list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
- /* skip root? */
- if (!destroy_root && blkg->blkcg == &blkio_root_cgroup)
- continue;
-
- /*
- * If cgroup removal path got to blk_group first
- * and removed it from cgroup list, then it will
- * take care of destroying cfqg also.
- */
- if (!blkiocg_del_blkio_group(blkg))
- blkg_destroy(blkg);
- else
- done = false;
- }
+ spin_lock_irq(q->queue_lock);
- spin_unlock_irq(q->queue_lock);
+ list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
+ struct blkio_cgroup *blkcg = blkg->blkcg;
- /*
- * Group list may not be empty if we raced cgroup removal
- * and lost. cgroup removal is guaranteed to make forward
- * progress and retrying after a while is enough. This
- * ugliness is scheduled to be removed after locking
- * update.
- */
- if (done)
- break;
+ /* skip root? */
+ if (!destroy_root && blkg->blkcg == &blkio_root_cgroup)
+ continue;
- msleep(10); /* just some random duration I like */
+ spin_lock(&blkcg->lock);
+ blkg_destroy(blkg);
+ spin_unlock(&blkcg->lock);
}
+ spin_unlock_irq(q->queue_lock);
+
for (i = 0; i < BLKIO_NR_POLICIES; i++)
update_root_blkg(q, i);
}
@@ -1590,45 +1556,45 @@ static int blkiocg_populate(struct cgroup_subsys *subsys, struct cgroup *cgroup)
ARRAY_SIZE(blkio_files));
}
+/**
+ * blkiocg_pre_destroy - cgroup pre_destroy callback
+ * @subsys: cgroup subsys
+ * @cgroup: cgroup of interest
+ *
+ * This function is called when @cgroup is about to go away and responsible
+ * for shooting down all blkgs associated with @cgroup. 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 int blkiocg_pre_destroy(struct cgroup_subsys *subsys,
struct cgroup *cgroup)
{
struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgroup);
- unsigned long flags;
- struct blkio_group *blkg;
- struct request_queue *q;
rcu_read_lock();
+ spin_lock_irq(&blkcg->lock);
- do {
- spin_lock_irqsave(&blkcg->lock, flags);
+ while (!hlist_empty(&blkcg->blkg_list)) {
+ struct blkio_group *blkg = hlist_entry(blkcg->blkg_list.first,
+ struct blkio_group, blkcg_node);
+ struct request_queue *q = rcu_dereference(blkg->q);
- if (hlist_empty(&blkcg->blkg_list)) {
- spin_unlock_irqrestore(&blkcg->lock, flags);
- break;
+ if (spin_trylock(q->queue_lock)) {
+ blkg_destroy(blkg);
+ spin_unlock(q->queue_lock);
+ } else {
+ spin_unlock_irq(&blkcg->lock);
+ rcu_read_unlock();
+ cpu_relax();
+ rcu_read_lock();
+ spin_lock(&blkcg->lock);
}
+ }
- blkg = hlist_entry(blkcg->blkg_list.first, struct blkio_group,
- blkcg_node);
- q = rcu_dereference(blkg->q);
- __blkiocg_del_blkio_group(blkg);
-
- spin_unlock_irqrestore(&blkcg->lock, flags);
-
- /*
- * This blkio_group is being unlinked as associated cgroup is
- * going away. Let all the IO controlling policies know about
- * this event.
- */
- spin_lock(&blkio_list_lock);
- spin_lock_irqsave(q->queue_lock, flags);
- blkg_destroy(blkg);
- spin_unlock_irqrestore(q->queue_lock, flags);
- spin_unlock(&blkio_list_lock);
- } while (1);
-
+ spin_unlock_irq(&blkcg->lock);
rcu_read_unlock();
-
return 0;
}
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index 88b2c3b..bebc442 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -376,7 +376,6 @@ static inline void blkiocg_set_start_empty_time(struct blkio_group *blkg,
extern struct blkio_cgroup blkio_root_cgroup;
extern struct blkio_cgroup *cgroup_to_blkio_cgroup(struct cgroup *cgroup);
extern struct blkio_cgroup *task_blkio_cgroup(struct task_struct *tsk);
-extern int blkiocg_del_blkio_group(struct blkio_group *blkg);
extern struct blkio_group *blkg_lookup(struct blkio_cgroup *blkcg,
struct request_queue *q);
struct blkio_group *blkg_lookup_create(struct blkio_cgroup *blkcg,
@@ -412,9 +411,6 @@ cgroup_to_blkio_cgroup(struct cgroup *cgroup) { return NULL; }
static inline struct blkio_cgroup *
task_blkio_cgroup(struct task_struct *tsk) { return NULL; }
-static inline int
-blkiocg_del_blkio_group(struct blkio_group *blkg) { return 0; }
-
static inline struct blkio_group *blkg_lookup(struct blkio_cgroup *blkcg,
void *key) { return NULL; }
static inline void blkiocg_update_timeslice_used(struct blkio_group *blkg,
diff --git a/block/cfq.h b/block/cfq.h
index 5584e1b..c8b15ef 100644
--- a/block/cfq.h
+++ b/block/cfq.h
@@ -79,11 +79,6 @@ static inline void cfq_blkiocg_update_completion_stats(struct blkio_group *blkg,
direction, sync);
}
-static inline int cfq_blkiocg_del_blkio_group(struct blkio_group *blkg)
-{
- return blkiocg_del_blkio_group(blkg);
-}
-
#else /* CFQ_GROUP_IOSCHED */
static inline void cfq_blkiocg_update_io_add_stats(struct blkio_group *blkg,
struct blkio_policy_type *pol,
@@ -119,10 +114,5 @@ static inline void cfq_blkiocg_update_completion_stats(struct blkio_group *blkg,
struct blkio_policy_type *pol, uint64_t start_time,
uint64_t io_start_time, bool direction, bool sync) { }
-static inline int cfq_blkiocg_del_blkio_group(struct blkio_group *blkg)
-{
- return 0;
-}
-
#endif /* CFQ_GROUP_IOSCHED */
#endif
--
1.7.7.3
--
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