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Subject: Re: [PATCH] blkcg: handle dying request_queue when associating a blkg
To:     Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
        Josef Bacik <josef@toxicpanda.com>
Cc:     kernel-team@fb.com, linux-block@vger.kernel.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        Ming Lei <ming.lei@redhat.com>
References: <20181211230308.66276-1-dennis@kernel.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <217c7b15-d54a-b51d-8b1a-c2ad0d404721@kernel.dk>
Date:   Wed, 12 Dec 2018 17:43:58 -0700
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On 12/11/18 4:03 PM, Dennis Zhou wrote:
> Between v3 [1] and v4 [2] of the blkg association series, the
> association point moved from generic_make_request_checks(), which is
> called after the request enters the queue, to bio_set_dev(), which is when
> the bio is formed before submit_bio(). When the request_queue goes away,
> the blkgs supporting the request_queue are destroyed and then the
> q->root_blkg is set to %NULL.
> 
> This patch adds a %NULL check to blkg_tryget_closest() to prevent the
> NPE caused by the above. It also adds a guard to see if the
> request_queue is dying when creating a blkg to prevent creating a blkg
> for a dead request_queue.

Applied, thanks Dennis.

-- 
Jens Axboe