From: Tejun Heo <tj@kernel.org>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: axboe@kernel.dk, ctalbott@google.com, rni@google.com,
linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
containers@lists.linux-foundation.org
Subject: Re: [PATCH 07/11] blkcg: make request_queue bypassing on allocation
Date: Fri, 13 Apr 2012 14:38:52 -0700 [thread overview]
Message-ID: <20120413213852.GJ12233@google.com> (raw)
In-Reply-To: <20120413213344.GA1825@redhat.com>
On Fri, Apr 13, 2012 at 05:33:44PM -0400, Vivek Goyal wrote:
> On Fri, Apr 13, 2012 at 02:05:48PM -0700, Tejun Heo wrote:
> > On Fri, Apr 13, 2012 at 04:55:01PM -0400, Vivek Goyal wrote:
> > > But neither seems to be the case here. So to make sure that blkg_lookup()
> > > under rcu will see the updated value of queue flag (bypass), are we
> > > relying on the fact that caller should see the DEAD flag and not go
> > > ahead with blkg_lookup()? If yes, atleast it is not obivious.
> >
> > We're relying on the fact that it doesn't matter anymore because all
> > blkgs will be shoot down in queue cleanup path which goes through rcu
> > free, which is different from deactivating individual policies. It
> > indeed is subtle. Umm... this is starting to get ridiculous. Why the
> > hell was megaraid messing with so many queues anyways?
>
> Well, blkcg_deactivate_policy() frees the policy data in a non-rcu
> manner. So group is around but policy data is gone. So technically if some
> IO submitter does not see the queue bypass flag, he might still try to
> access blkg->pd[pol->plid] after being freed.
No, we always go through blkg_destroy_all() and each blkg along with
any attached policy_data will go through RCU grace period before
getting destroyed. It is stupid subtle but nevertheless correct.
> Having said that, in this case we are probably fine as blk_release_queue()
> is executed after last reference to queue is dropped and no more IO can
> come. May be a 2 line comment will help.
Yeah, we're guaranteed that by the time blk_release_queue() executes
nobody is traversing the queue. Hmmm... right, this is much easier to
wrap one's head around. I'll use this explanation in the comment.
> BTW, looks like blkio_exit_group_fn() probably is not a good name anymore
> as it is not even called when policy is being deactivated. It should
> probably be now .blkio_exit_policy_data_fn() or something like that.
Heh, I'm brewing mass blkcg API rename patch as we speak.
Thanks.
--
tejun
next prev parent reply other threads:[~2012-04-13 21:38 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-13 20:11 [PATCHSET] block: per-queue policy activation, take#2 Tejun Heo
2012-04-13 20:11 ` [PATCH 01/11] cfq: fix build breakage & warnings Tejun Heo
2012-04-13 20:11 ` [PATCH 02/11] blkcg: kill blkio_list and replace blkio_list_lock with a mutex Tejun Heo
2012-04-13 20:11 ` [PATCH 03/11] blkcg: use @pol instead of @plid in update_root_blkg_pd() and blkcg_print_blkgs() Tejun Heo
2012-04-13 20:11 ` [PATCH 04/11] blkcg: remove static policy ID enums Tejun Heo
2012-04-13 20:11 ` [PATCH 05/11] blkcg: make blkg_conf_prep() take @pol and return with queue lock held Tejun Heo
2012-04-13 20:11 ` [PATCH 06/11] blkcg: make sure blkg_lookup() returns %NULL if @q is bypassing Tejun Heo
2012-04-13 21:50 ` [PATCH UPDATED " Tejun Heo
2012-04-13 20:11 ` [PATCH 07/11] blkcg: make request_queue bypassing on allocation Tejun Heo
2012-04-13 20:32 ` Vivek Goyal
2012-04-13 20:37 ` Tejun Heo
2012-04-13 20:44 ` Vivek Goyal
2012-04-13 20:47 ` Tejun Heo
2012-04-13 20:55 ` Vivek Goyal
2012-04-13 21:05 ` Tejun Heo
2012-04-13 21:16 ` Tejun Heo
2012-04-13 21:31 ` Tejun Heo
2012-04-17 12:04 ` James Bottomley
2012-04-18 21:42 ` Tejun Heo
2012-04-13 21:33 ` Vivek Goyal
2012-04-13 21:38 ` Tejun Heo [this message]
2012-04-16 12:39 ` Vivek Goyal
2012-04-13 20:37 ` Vivek Goyal
2012-04-13 20:11 ` [PATCH 08/11] blkcg: add request_queue->root_blkg Tejun Heo
2012-04-13 20:11 ` [PATCH 09/11] blkcg: implement per-queue policy activation Tejun Heo
2012-04-13 20:11 ` [PATCH 10/11] blkcg: drop stuff unused after per-queue policy activation update Tejun Heo
2012-04-13 20:11 ` [PATCH 11/11] blkcg: shoot down blkgs if all policies are deactivated Tejun Heo
2012-04-20 8:09 ` [PATCHSET] block: per-queue policy activation, take#2 Jens Axboe
2012-04-20 12:02 ` Jens Axboe
2012-04-20 17:17 ` Tejun Heo
2012-04-20 19:08 ` Jens Axboe
2012-04-25 18:19 ` Tejun Heo
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