linux-kernel - Re: [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint

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Message-ID: <1540487511.66186.31.camel@acm.org>
Date:   Thu, 25 Oct 2018 10:11:51 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Johannes Berg <johannes@...solutions.net>,
        Tejun Heo <tj@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "tytso@....edu" <tytso@....edu>
Subject: Re: [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep
 complaint

On Thu, 2018-10-25 at 19:02 +0200, Johannes Berg wrote:
> On Thu, 2018-10-25 at 15:05 +0000, Bart Van Assche wrote:
> > It can happen that the direct I/O queue creates and destroys an empty
> > workqueue from inside a work function.
> 
> So, thinking about this more, can you guarantee (somehow) that the
> workqueue is empty at this point?

In general, no. But for the direct I/O case this can be guaranteed. Please
have a look at the code in sb_init_dio_done_wq() if you would not yet have
done this.

> Do you know how to reproduce this?

The lockdep complaint in the patch description is easy to reproduce. The
way I reproduce it is as follows:

git clone https://github.com/osandov/blktests
(cd blktests && ./check -q nvmeof-mp)

Bart.