linux-kernel - Re: [GIT PULL] io_uring fixes for 5.6-rc

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Date:   Fri, 13 Mar 2020 13:18:30 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jens Axboe <axboe@...com>, "Paul E. McKenney" <paulmck@...nel.org>,
        Tejun Heo <tj@...nel.org>
Cc:     io-uring <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] io_uring fixes for 5.6-rc

On Fri, Mar 13, 2020 at 10:50 AM Jens Axboe <axboe@...com> wrote:
>
> Just a single fix here, improving the RCU callback ordering from last
> week. After a bit more perusing by Paul, he poked a hole in the
> original.

Ouch.

If I read this patch correctly, you're now adding a rcu_barrier() onto
the system workqueue for each io_uring context freeing op.

This makes me worry:

 - I think system_wq is unordered, so does it even guarantee that the
rcu_barrier happens after whatever work you're expecting it to be
after?

Or is it using a workqueue not because it wants to serialize with any
other work, but because it needs to use rcu_barrier in a context where
it can't sleep?

But the commit message does seem to imply that ordering is important..

 - doesn't this have the potential to flood the system_wq be full of
flushing things that all could take a while..

I've pulled it, and it may all be correct, just chalk this message up
to "Linus got nervous looking at it".

Added Paul and Tejun to the participants explicitly.

             Linus