linux-ext4 - Re: INFO: task hung in blk_queue

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Message-ID: <20180602045353.GA20257@ming.t460p>
Date:   Sat, 2 Jun 2018 12:54:02 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Bart.VanAssche@....com, dvyukov@...gle.com,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        jthumshirn@...e.de, alan.christopher.jenkins@...il.com,
        syzbot+c4f9cebf9d651f6e54de@...kaller.appspotmail.com,
        martin.petersen@...cle.com, dan.j.williams@...el.com, hch@....de,
        oleksandr@...alenko.name, martin@...htvoll.de, hare@...e.com,
        syzkaller-bugs@...glegroups.com, ross.zwisler@...ux.intel.com,
        keith.busch@...el.com, linux-ext4@...r.kernel.org
Subject: Re: INFO: task hung in blk_queue_enter

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