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Date:   Wed, 21 Aug 2019 10:45:05 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Bart Van Assche <bvanassche@acm.org>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        Greg KH <gregkh@linuxfoundation.org>,
        Mike Snitzer <snitzer@redhat.com>
Subject: Re: [PATCH] block: don't acquire .sysfs_lock before removing mq &
 iosched kobjects
Message-ID: <20190821024503.GC24167@ming.t460p>
References: <20190816135506.29253-1-ming.lei@redhat.com>
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 <20190819081536.GA9852@ming.t460p>
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On Tue, Aug 20, 2019 at 02:07:43PM -0700, Bart Van Assche wrote:
> On 8/19/19 1:15 AM, Ming Lei wrote:
> > hctx->tags is tagset wide or host-wide, which is protected by set->tag_list_lock.
> 
> Isn't the purpose of set->tag_list_lock to protect set->tag_list?

In theory, .tags is shared among all queues in tagset, so it should be
protected by tagset wide lock, what is why set->tag_list_lock is held
in blk_mq_update_nr_hw_queues().

However, both sysfs and debugfs read/write doesn't hold this lock because
the .tags won't be shrunk in blk_mq_update_nr_requests(), and
blk_mq_update_nr_hw_queues un-registers debugfs/sysfs before changing .tags.


thanks,
Ming