release_stripe still has big lock contention. We just add the stripe to a llist
without taking device_lock. We let the raid5d thread to do the real stripe
release, which must hold device_lock anyway. In this way, release_stripe
doesn't hold any locks.

The side effect is the released stripes order is changed. But sounds not a big
deal, stripes are never handled in order. And I thought block layer can already
do nice request merge, which means order isn't that important.

I kept the unplug release batch, which is unnecessary with this patch from lock
contention avoid point of view, and actually if we delete it, the stripe_head
release_list and lru can share storage. But the unplug release batch is also
helpful for request merge. We probably can delay wakeup raid5d till unplug, but
I'm still afraid of the case which raid5d is running.

Signed-off-by: Shaohua Li <redacted>
---
 drivers/md/raid5.c |   53 ++++++++++++++++++++++++++++++++++++++++++++++++-----
 drivers/md/raid5.h |    3 +++
 2 files changed, 51 insertions(+), 5 deletions(-)

Index: linux/drivers/md/raid5.c
===================================================================

--- linux.orig/drivers/md/raid5.c       2013-03-18 08:49:43.276628437 +0800
+++ linux/drivers/md/raid5.c    2013-03-18 10:33:58.561988946 +0800

@@ -262,12 +262,45 @@ static void __release_stripe(struct r5co
                do_release_stripe(conf, sh);
 }

+/* should hold conf->device_lock already */
+static int release_stripe_list(struct r5conf *conf)
+{
+       struct stripe_head *sh;
+       struct llist_node *node;
+       int count = 0;
+
+       while (1) {
+               node = llist_del_first(&conf->released_stripes);
+               if (!node)
+                       break;
+               sh = llist_entry(node, struct stripe_head, release_list);