linux-kernel - Re: [PATCH 0/3] fix blkcg offlining and destruction

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Message-ID: <20180831222412.GA1488037@devbig004.ftw2.facebook.com>
Date:   Fri, 31 Aug 2018 15:24:12 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Dennis Zhou <dennisszhou@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Johannes Weiner <hannes@...xchg.org>,
        Josef Bacik <josef@...icpanda.com>, kernel-team@...com,
        linux-block@...r.kernel.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] fix blkcg offlining and destruction

On Fri, Aug 31, 2018 at 04:22:41PM -0400, Dennis Zhou wrote:
> Hi everyone,
> 
> This is a split of an earlier series I sent out [1] containing the first
> 3 patches with fixes from feedback. This series tackles the first
> problem where blkcgs were not being destroyed.
> 
> There is a regression in blkcg destruction where references weren't
> properly put causing blkcgs to never be destroyed. Previously, blkgs
> were destroyed during offlining of the blkcg. This puts back the blkcg
> reference a blkg holds allowing blkcg ref to reach zero. Then,
> blkcg_css_free() is called as part of the final cleanup.
> 
> To address the problem, 0001 reverts the broken commit, 0002 delays
> blkg destruction until writeback has finished, and 0003 closes the
> window on a race condition between a css migration and dying, and
> blkg association. This should fix the issue where blkg_get() was getting
> called when a blkcg had already begun exiting. If a bio finds itself
> here, it will just fall back to root. Oddly enough at one point,
> blk-throttle was using policy data from and associating with potentially
> different blkgs, thus how this was exposed.

For patches 1-3,

  Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun