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blkcg: make sure blkg_lookup() returns %NULL if @q is bypassing

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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@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Tejun Heo 2012-04-13 14:50:53 -07:00 committed by Jens Axboe
parent da8b066262
commit 80fd99792b
2 changed files with 46 additions and 19 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

50
block/blk-cgroup.c
View file

@ -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 @@ struct blkio_group *blkg_lookup_create(struct blkio_cgroup *blkcg,
}
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;

15
block/blk-core.c
View file

@ -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);