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Subject: Re: INFO: task hung in blk_queue_enter
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
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        oleksandr@natalenko.name, martin@lichtvoll.de, hare@suse.com,
        syzkaller-bugs@googlegroups.com, ross.zwisler@linux.intel.com,
        keith.busch@intel.com, linux-ext4@vger.kernel.org
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <f43465cf-e2f1-807f-d283-89033c056b5b@kernel.dk>
Date:   Fri, 1 Jun 2018 22:31:13 -0600
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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