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Subject: Re: [PATCH block/for-5.4-fixes] blkcg: make blkcg_print_stat() print
 stats only for online blkgs
To:     Tejun Heo <tj@kernel.org>
Cc:     Roman Gushchin <guro@fb.com>, linux-block@vger.kernel.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, Josef Bacik <jbacik@fb.com>
References: <20191105160951.GS3622521@devbig004.ftw2.facebook.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <1ef277bb-8591-6029-2e9b-319820b3460c@kernel.dk>
Date:   Wed, 6 Nov 2019 17:08:59 -0700
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On 11/5/19 9:09 AM, Tejun Heo wrote:
> blkcg_print_stat() iterates blkgs under RCU and doesn't test whether
> the blkg is online.  This can call into pd_stat_fn() on a pd which is
> still being initialized leading to an oops.
> 
> The heaviest operation - recursively summing up rwstat counters - is
> already done while holding the queue_lock.  Expand queue_lock to cover
> the other operations and skip the blkg if it isn't online yet.  The
> online state is protected by both blkcg and queue locks, so this
> guarantees that only online blkgs are processed.

Applied, thanks Tejun.

-- 
Jens Axboe