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

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Date:   Fri, 1 Jun 2018 22:31:13 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Ming Lei <ming.lei@...hat.com>
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 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.

> 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.

-- 
Jens Axboe