From: Ming Lei <ming.lei@redhat.com>
Subject: Re: INFO: task hung in blk_queue_enter
Date: Sat, 2 Jun 2018 12:54:02 +0800
Message-ID: <20180602045353.GA20257@ming.t460p>
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Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        Bart.VanAssche@wdc.com, dvyukov@google.com,
        linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
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To: Jens Axboe <axboe@kernel.dk>
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On Fri, Jun 01, 2018 at 10:31:13PM -0600, Jens Axboe wrote:
> On 6/1/18 8:36 PM, Ming Lei wrote:
> >> clear the bit when we need to, potentially even adding a debug check to
> >> blk_queue_enter_live() that complains if the ref was not already
> >> elevated. Though that would be expensive, compared to the percpu inc
> >> now. Not saying the bit is necessarily the best way forward, but I do
> >> like it a LOT more than adding an argument to generic_make_request.
> > 
> > We may introduce generic_make_request_with_ref() for blk_queue_split(),
> > what do you think of the following patch?
> 
> Functionally it'll work as well.

If you are fine with this one, I'm glad to prepare for a normal one,
so that we can save one precious bio flag, and the total number is
just 13.

> 
> > In DM or MD, there is the following use case:
> > 
> > 1) bio comes from fs, then one clone is made from this bio
> > 
> > 2) the cloned bio is submitted via generic_make_request() from
> > another context, such as wq.
> > 
> > 3) then the flag of BIO_QUEUE_ENTERED is inherited and causes
> > the queue usage counter isn't held.
> 
> We don't copy flags on a clone.

Yeah, you are right, seems I ignored that.

Thanks,
Ming