linux-kernel - Re: dm crypt: initialize tasklet in crypt_io

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Date:   Tue, 7 Mar 2023 10:12:06 +0800
From:   Hou Tao <houtao@...weicloud.com>
To:     Mike Snitzer <snitzer@...nel.org>
Cc:     dm-devel@...hat.com, Alasdair Kergon <agk@...hat.com>,
        Ignat Korchagin <ignat@...udflare.com>,
        linux-kernel@...r.kernel.org, houtao1@...wei.com
Subject: Re: dm crypt: initialize tasklet in crypt_io_init()

Hi,

On 3/7/2023 3:31 AM, Mike Snitzer wrote:
> On Mon, Mar 06 2023 at  8:49P -0500,
> Hou Tao <houtao@...weicloud.com> wrote:
>
>> From: Hou Tao <houtao1@...wei.com>
>>
>> When neither no_read_workqueue nor no_write_workqueue are enabled,
>> tasklet_trylock() in crypt_dec_pending() may still return false due to
>> an uninitialized state, and dm-crypt will do io completion in io_queue
>> instead of current context unnecessarily.
> Have you actually experienced this?
Yes. I had written a bpftrace script to check the completion context of
blkdev_bio_end_io_simple() when doing direct io read on dm-crypt device. The
expected context should be unbound workers of crypt_queue, but sometimes the
context is the bound worker of io_queue.
>
>> Fix it by initializing io->tasklet in crypt_io_init().
> Really would rather avoid always calling tasklet_init(). But I can
> optimize it away with a later patch.
My first though was "io->tasklet.state = 0", but it may be fragile because it
operated on the internal status of tasklet, so I switch to tasklet_init().
>
> Mike