blkcg: move bulk of blkcg_gq release operations to the RCU callback
Currently, when the last reference of a blkcg_gq is put, all then
release operations sans the actual freeing happen directly in
blkg_put(). As blkg_put() may be called under queue_lock, all
pd_exit_fn()s may be too. This makes it impossible for pd_exit_fn()s
to use del_timer_sync() on timers which grab the queue_lock which is
an irq-safe lock due to the deadlock possibility described in the
comment on top of del_timer_sync().
This can be easily avoided by perfoming the release operations in the
RCU callback instead of directly from blkg_put(). This patch moves
the blkcg_gq release operations to the RCU callback.
As this leaves __blkg_release() with only call_rcu() invocation,
blkg_rcu_free() is renamed to __blkg_release_rcu(), exported and
call_rcu() invocation is now done directly from blkg_put() instead of
going through __blkg_release() which is removed.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 6bbe0a9..d074760 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -369,13 +369,17 @@
q->root_rl.blkg = NULL;
}
-static void blkg_rcu_free(struct rcu_head *rcu_head)
+/*
+ * A group is RCU protected, but having an rcu lock does not mean that one
+ * can access all the fields of blkg and assume these are valid. For
+ * example, don't try to follow throtl_data and request queue links.
+ *
+ * Having a reference to blkg under an rcu allows accesses to only values
+ * local to groups like group stats and group rate limits.
+ */
+void __blkg_release_rcu(struct rcu_head *rcu_head)
{
- blkg_free(container_of(rcu_head, struct blkcg_gq, rcu_head));
-}
-
-void __blkg_release(struct blkcg_gq *blkg)
-{
+ struct blkcg_gq *blkg = container_of(rcu_head, struct blkcg_gq, rcu_head);
int i;
/* tell policies that this one is being freed */
@@ -388,21 +392,15 @@
/* release the blkcg and parent blkg refs this blkg has been holding */
css_put(&blkg->blkcg->css);
- if (blkg->parent)
+ if (blkg->parent) {
+ spin_lock_irq(blkg->q->queue_lock);
blkg_put(blkg->parent);
+ spin_unlock_irq(blkg->q->queue_lock);
+ }
- /*
- * A group is freed in rcu manner. But having an rcu lock does not
- * mean that one can access all the fields of blkg and assume these
- * are valid. For example, don't try to follow throtl_data and
- * request queue links.
- *
- * Having a reference to blkg under an rcu allows acess to only
- * values local to groups like group stats and group rate limits
- */
- call_rcu(&blkg->rcu_head, blkg_rcu_free);
+ blkg_free(blkg);
}
-EXPORT_SYMBOL_GPL(__blkg_release);
+EXPORT_SYMBOL_GPL(__blkg_release_rcu);
/*
* The next function used by blk_queue_for_each_rl(). It's a bit tricky
