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

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Mar 13, 2020 at 10:50 AM Jens Axboe <axboe@fb.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