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    SubjectRe: [PATCH 2/9] blkcg: drop unnecessary RCU locking
    On Fri, Feb 17, 2012 at 09:07:13AM -0800, Tejun Heo wrote:
    > 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.

    BTW, the reason this doesn't work is due to the broken percpu
    allocation in blkg_alloc(). We should remove unnecessary rcu lockings
    after updating stat allocation.

    Thanks.

    --
    tejun


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