From 9f13ef678efd977487fc0c2e489f17c9a8c67a3e Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Mon, 5 Mar 2012 13:15:21 -0800 Subject: [PATCH] blkcg: use double locking instead of RCU for blkg synchronization blkgs are chained from both blkcgs and request_queues and thus subjected to two locks - blkcg->lock and q->queue_lock. As both blkcg and q can go away anytime, locking during removal is tricky. It's currently solved by wrapping removal inside RCU, which makes the synchronization complex. There are three locks to worry about - the outer RCU, q lock and blkcg lock, and it leads to nasty subtle complications like conditional synchronize_rcu() on queue exit paths. For all other paths, blkcg lock is naturally nested inside q lock and the only exception is blkcg removal path, which is a very cold path and can be implemented as clumsy but conceptually-simple reverse double lock dancing. This patch updates blkg removal path such that blkgs are removed while holding both q and blkcg locks, which is trivial for request queue exit path - blkg_destroy_all(). The blkcg removal path, blkiocg_pre_destroy(), implements reverse double lock dancing essentially identical to ioc_release_fn(). This simplifies blkg locking - no half-dead blkgs to worry about. Now unnecessary RCU annotations will be removed by the next patch. Signed-off-by: Tejun Heo Cc: Vivek Goyal Signed-off-by: Jens Axboe --- Reading git-format-patch failed