Messages in this thread | | | Date | Fri, 17 Feb 2012 09:07:13 -0800 | From | Tejun Heo <> | Subject | Re: [PATCH 2/9] blkcg: drop unnecessary RCU locking |
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Hey, Vivek.
On Fri, Feb 17, 2012 at 11:19:58AM -0500, Vivek Goyal wrote: > On Thu, Feb 16, 2012 at 02:37:51PM -0800, Tejun Heo wrote: > > Now that blkg additions / removals are always done under both q and > > blkcg locks, the only place RCU locking is used is blkg_lookup() for > > lockless lookup. This patch drops unncessary RCU locking replacing it > > with plain blkcg / q locking as necessary. > > > > * blkg_lookup_create() and blkiocg_pre_destroy() already perform > > proper locking and don't need RCU. Dropped. > > But blkg_lookup_create() is called under rcu() to protect blkcg pointer. > And blkg_lookup() is also happening under same rcu read lock. So I think > you can't drop rcu from blkg_lookup_create().
Ooh, right. Will fix.
> > { > > struct blkio_group *blkg, *new_blkg; > > > > - WARN_ON_ONCE(!rcu_read_lock_held()); > > Don't we want to be called with rcu lock held needed for blkg_lookup()?
We want WARN_ON_ONCE(none of RCU, blkcg and queue lock held). We can do it using lockdep macros inside #ifdef CONFIG_LOCKDEP. It's just a bit clunky.
Thanks.
-- tejun
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