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SubjectRe: [PATCH 2/9] blkcg: drop unnecessary RCU locking
On Fri, Feb 17, 2012 at 01:08:00PM -0500, Vivek Goyal wrote:
> > Modifying blkgs require both blkcg and queue locks,
> > so read access can be done holding any lock.
>
> This is the point I am not getting. How blkg_lookup() is safe just
> under queue lock. What stops freeing up blkg associated with other
> queues. I thought caller needs to hold rcu_read_lock() also to
> make sure it can safely compare blkg->q == q and return the blkg
> belonging to the queue in question.

Ooh, you're right. I got confused. We should be holding either blkcg
lock or rcu_read_lock() across blkg_lookup(). Will update.

Thanks.

--
tejun


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