Messages in this thread |  | | | Date | Thu, 19 Jan 2012 10:54:45 -0500 | | From | Vivek Goyal <> | | Subject | Re: [PATCH 01/12] blkcg: obtaining blkg should be enclosed inside rcu_read_lock() |
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On Thu, Jan 19, 2012 at 07:39:38AM -0800, Tejun Heo wrote: > Hello, Vivek. > > On Thu, Jan 19, 2012 at 05:07:29AM -0500, Vivek Goyal wrote: > > On Wed, Jan 18, 2012 at 05:11:19PM -0800, Tejun Heo wrote: > > > When looking up or creating blkg's, both blk-throttle and cfq-iosched > > > drops rcu_read_lock() right after lookup is complete. This isn't > > > safe. Refcnt isn't incremented at that point and rcu lock is the only > > > thing holding the blkg. It shouldn't be dropped until after refcnt is > > > incremented by the caller. > > > > throtl_get_tg() and cfq_get_cfqg() are called with queue lock held and > > tg and cfqg are protected by queue lock as they can not go away as long > > as queue lock is held. > > Ah, right. > > > I had used rcu read lock to access blkcg pointer here. That's why when > > we are done with accessing blkcg, we drop rcu read lock and return back > > to caller with group pointer, which is aready holding either a queue > > lock or rcu read lock to protect returned group pointer. > > > > So if we are protecting blkcg using rcu, then it should make sense to > > take that lock inside throtl_get_tg() and cfq_get_cfqg() respectively and > > it should not be left to the caller? > > No, no matter whatever synchronization scheme is in use, the code is > seriously screwed up if it's doing something like, > > lock(); > a = lookup(); > unlock(); > return a; > > You should *NEVER* be doing that.
I guess ioc_lookup_icq() is doing something similar. We call it under queue lock. Take rcu lock inside for sanity of radix tree and then release rcu lock and return icq.
Thanks Vivek
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