linux-kernel - Re: [PATCH v2] blk-cgroup: remove entries in blkg

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Date:   Wed, 11 Apr 2018 17:26:12 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "tj@...nel.org" <tj@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "nborisov@...e.com" <nborisov@...e.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>, "shli@...com" <shli@...com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "00moses.alexander00@...il.com" <00moses.alexander00@...il.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "joseph.qi@...ux.alibaba.com" <joseph.qi@...ux.alibaba.com>
Subject: Re: [PATCH v2] blk-cgroup: remove entries in blkg_tree before queue
 release

On Wed, 2018-04-11 at 10:15 -0700, tj@...nel.org wrote:
> On Wed, Apr 11, 2018 at 05:06:41PM +0000, Bart Van Assche wrote:
> > A simple and effective solution is to dissociate a request queue from the
> > block cgroup controller before blk_cleanup_queue() returns. This is why commit
> > a063057d7c73 ("block: Fix a race between request queue removal and the block
> > cgroup controller") moved the blkcg_exit_queue() call from __blk_release_queue()
> > into blk_cleanup_queue().
> 
> which is broken.  We can try to switch the lifetime model to revoking
> all live objects but that likely is a lot more involving than blindly
> moving blkg shootdown from release to cleanup.  Implementing sever
> semantics is usually a lot more involved / fragile because it requires
> explicit participation from all users (exactly the same way revoking
> ->queue_lock is difficult).
> 
> I'm not necessarily against switching to sever model, but what the
> patch did isn't that.  It just moved some code without actually
> understanding or auditing what the implications are.

Hello Tejun,

Please explain what you wrote further. It's not clear to me why you think
that anything is broken nor what a "sever model" is.

I think we really need the blkcg_exit_queue() call in blk_cleanup_queue()
to avoid race conditions between request queue cleanup and the block cgroup
controller. Additionally, since it is guaranteed that no new requests will
be submitted to a queue after it has been marked dead I don't see why it
would make sense to keep the association between a request queue and the
blkcg controller until the last reference on a queue is dropped.

Bart.