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authorTejun Heo <tj@kernel.org>2012-03-06 01:15:21 +0400
committerJens Axboe <axboe@kernel.dk>2012-03-07 00:27:24 +0400
commit9f13ef678efd977487fc0c2e489f17c9a8c67a3e (patch)
treee58a2dd153ad24b2ea173d5dfb575c507e1f7589 /block/blk-throttle.c
parente8989fae38d9831c72b20375a206a919ca468c52 (diff)
downloadlinux-9f13ef678efd977487fc0c2e489f17c9a8c67a3e.tar.xz
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 <tj@kernel.org> Cc: Vivek Goyal <vgoyal@redhat.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block/blk-throttle.c')
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