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Date: Fri, 13 Apr 2012 14:50:53 -0700
From: Tejun Heo <tj@...nel.org>
To: axboe@...nel.dk
Cc: vgoyal@...hat.com, ctalbott@...gle.com, rni@...gle.com,
linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
containers@...ts.linux-foundation.org
Subject: [PATCH UPDATED 06/11] blkcg: make sure blkg_lookup() returns %NULL
if @q is bypassing
Currently, blkg_lookup() doesn't check @q bypass state. This patch
updates blk_queue_bypass_start() to do synchronize_rcu() before
returning and updates blkg_lookup() to check blk_queue_bypass() and
return %NULL if bypassing. This ensures blkg_lookup() returns %NULL
if @q is bypassing.
This is to guarantee that nobody is accessing policy data while @q is
bypassing, which is necessary to allow replacing blkio_cgroup->pd[] in
place on policy [de]activation.
v2: Added more comments explaining bypass guarantees as suggested by
Vivek.
v3: Added more comments explaining why there's no synchronize_rcu() in
blk_cleanup_queue() as suggested by Vivek.
Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Vivek Goyal <vgoyal@...hat.com>
---
This update only adds some comments. It does cause some offset
warnings for the following patches but nothing patch(1) can't handle.
The git branch has been updated accordingly.
Thanks.
block/blk-cgroup.c | 50 +++++++++++++++++++++++++++++++++-----------------
block/blk-core.c | 15 +++++++++++++--
2 files changed, 46 insertions(+), 19 deletions(-)
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index f6581a0..d6e4555 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -137,6 +137,38 @@ static struct blkio_group *blkg_alloc(struct blkio_cgroup *blkcg,
return blkg;
}
+static struct blkio_group *__blkg_lookup(struct blkio_cgroup *blkcg,
+ struct request_queue *q)
+{
+ struct blkio_group *blkg;
+ struct hlist_node *n;
+
+ hlist_for_each_entry_rcu(blkg, n, &blkcg->blkg_list, blkcg_node)
+ if (blkg->q == q)
+ return blkg;
+ return NULL;
+}
+
+/**
+ * blkg_lookup - lookup blkg for the specified blkcg - q pair
+ * @blkcg: blkcg of interest
+ * @q: request_queue of interest
+ *
+ * Lookup blkg for the @blkcg - @q pair. This function should be called
+ * under RCU read lock and is guaranteed to return %NULL if @q is bypassing
+ * - see blk_queue_bypass_start() for details.
+ */
+struct blkio_group *blkg_lookup(struct blkio_cgroup *blkcg,
+ struct request_queue *q)
+{
+ WARN_ON_ONCE(!rcu_read_lock_held());
+
+ if (unlikely(blk_queue_bypass(q)))
+ return NULL;
+ return __blkg_lookup(blkcg, q);
+}
+EXPORT_SYMBOL_GPL(blkg_lookup);
+
struct blkio_group *blkg_lookup_create(struct blkio_cgroup *blkcg,
struct request_queue *q,
bool for_root)
@@ -150,13 +182,11 @@ struct blkio_group *blkg_lookup_create(struct blkio_cgroup *blkcg,
/*
* This could be the first entry point of blkcg implementation and
* we shouldn't allow anything to go through for a bypassing queue.
- * The following can be removed if blkg lookup is guaranteed to
- * fail on a bypassing queue.
*/
if (unlikely(blk_queue_bypass(q)) && !for_root)
return ERR_PTR(blk_queue_dead(q) ? -EINVAL : -EBUSY);
- blkg = blkg_lookup(blkcg, q);
+ blkg = __blkg_lookup(blkcg, q);
if (blkg)
return blkg;
@@ -185,20 +215,6 @@ out:
}
EXPORT_SYMBOL_GPL(blkg_lookup_create);
-/* called under rcu_read_lock(). */
-struct blkio_group *blkg_lookup(struct blkio_cgroup *blkcg,
- struct request_queue *q)
-{
- struct blkio_group *blkg;
- struct hlist_node *n;
-
- hlist_for_each_entry_rcu(blkg, n, &blkcg->blkg_list, blkcg_node)
- if (blkg->q == q)
- return blkg;
- return NULL;
-}
-EXPORT_SYMBOL_GPL(blkg_lookup);
-
static void blkg_destroy(struct blkio_group *blkg)
{
struct request_queue *q = blkg->q;
diff --git a/block/blk-core.c b/block/blk-core.c
index 991c1d6..f2db628 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -416,7 +416,8 @@ void blk_drain_queue(struct request_queue *q, bool drain_all)
* In bypass mode, only the dispatch FIFO queue of @q is used. This
* function makes @q enter bypass mode and drains all requests which were
* throttled or issued before. On return, it's guaranteed that no request
- * is being throttled or has ELVPRIV set.
+ * is being throttled or has ELVPRIV set and blk_queue_bypass() %true
+ * inside queue or RCU read lock.
*/
void blk_queue_bypass_start(struct request_queue *q)
{
@@ -426,6 +427,8 @@ void blk_queue_bypass_start(struct request_queue *q)
spin_unlock_irq(q->queue_lock);
blk_drain_queue(q, false);
+ /* ensure blk_queue_bypass() is %true inside RCU read lock */
+ synchronize_rcu();
}
EXPORT_SYMBOL_GPL(blk_queue_bypass_start);
@@ -462,7 +465,15 @@ void blk_cleanup_queue(struct request_queue *q)
spin_lock_irq(lock);
- /* dead queue is permanently in bypass mode till released */
+ /*
+ * Dead queue is permanently in bypass mode till released. Note
+ * that, unlike blk_queue_bypass_start(), we aren't performing
+ * synchronize_rcu() after entering bypass mode to avoid the delay
+ * as some drivers create and destroy a lot of queues while
+ * probing. This is still safe because blk_release_queue() will be
+ * called only after the queue refcnt drops to zero and nothing,
+ * RCU or not, would be traversing the queue by then.
+ */
q->bypass_depth++;
queue_flag_set(QUEUE_FLAG_BYPASS, q);
--
1.7.7.3
--
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