[PATCH 09/13] blk-throttle: Free up a group only after one rcu grace period

Vivek Goyal
2011-05-19T19:39:05Z

Soon we will allow accessing a throtl_grp under rcu_read_lock(). Hence
start freeing up throtl_grp after one rcu grace period.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
block/blk-throttle.c | 22 +++++++++++++++++++++-
1 files changed, 21 insertions(+), 1 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index b9412d1..90ad407 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -78,6 +78,8 @@ struct throtl_grp {

/* Some throttle limits got updated for the group */
int limits_changed;
+
+ struct rcu_head rcu_head;
};

struct throtl_data
@@ -151,12 +153,30 @@ static inline struct throtl_grp *throtl_ref_get_tg(struct throtl_grp *tg)
return tg;
}

+static void throtl_free_tg(struct rcu_head *head)
+{
+ struct throtl_grp *tg;
+
+ tg = container_of(head, struct throtl_grp, rcu_head);
+ kfree(tg);
+}
+
static void throtl_put_tg(struct throtl_grp *tg)
{
BUG_ON(atomic_read(&tg->ref) <= 0);
if (!atomic_dec_and_test(&tg->ref))
return;
- kfree(tg);
+
+ /*
+ * 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(&tg->rcu_head, throtl_free_tg);
}

static void throtl_init_group(struct throtl_grp *tg)
--
1.7.4.4