Re: [PATCH] io_uring/io-wq: don't clear PF_IO_WORKER on exit

[Zorro Lang <zlang@redhat.com>]

On Tue, Jun 13, 2023 at 07:14:25PM -0600, Jens Axboe wrote:
> On 6/13/23 6:54?PM, Zorro Lang wrote:
> > On Mon, Jun 12, 2023 at 12:11:57PM -0600, Jens Axboe wrote:
> >> A recent commit gated the core dumping task exit logic on current->flags
> >> remaining consistent in terms of PF_{IO,USER}_WORKER at task exit time.
> >> This exposed a problem with the io-wq handling of that, which explicitly
> >> clears PF_IO_WORKER before calling do_exit().
> >>
> >> The reasons for this manual clear of PF_IO_WORKER is historical, where
> >> io-wq used to potentially trigger a sleep on exit. As the io-wq thread
> >> is exiting, it should not participate any further accounting. But these
> >> days we don't need to rely on current->flags anymore, so we can safely
> >> remove the PF_IO_WORKER clearing.
> >>
> >> Reported-by: Zorro Lang <zlang@redhat.com>
> >> Reported-by: Dave Chinner <david@fromorbit.com>
> >> Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
> >> Link: https://lore.kernel.org/all/ZIZSPyzReZkGBEFy@dread.disaster.area/
> >> Fixes: f9010dbdce91 ("fork, vhost: Use CLONE_THREAD to fix freezer/ps regression")
> >> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> >>
> >> ---
> > 
> > Hi,
> > 
> > This patch fix the issue I reported. The bug can be reproduced on v6.4-rc6,
> > then test passed on v6.4-rc6 with this patch.
> > 
> > But I found another KASAN bug [1] on aarch64 machine, by running generic/388.
> > I hit that 3 times. And hit a panic [2] (once after that kasan bug) on a x86_64
> > with pmem device (mount with dax=never), by running geneirc/388 too.
> 
> Can you try with this? I suspect the preempt dance isn't really
> necessary, but I can't quite convince myself that it isn't. In any case,
> I think this should fix it and this was exactly what I was worried about
> but apparently not able to easily trigger or prove...
> 
> 
> diff --git a/io_uring/io-wq.c b/io_uring/io-wq.c
> index fe38eb0cbc82..878ec3feeba9 100644
> --- a/io_uring/io-wq.c
> +++ b/io_uring/io-wq.c
> @@ -220,7 +220,9 @@ static void io_worker_exit(struct io_worker *worker)
>  	list_del_rcu(&worker->all_list);
>  	raw_spin_unlock(&wq->lock);
>  	io_wq_dec_running(worker);
> -	worker->flags = 0;
> +	preempt_disable();
> +	current->worker_private = NULL;
> +	preempt_enable();

Hi,

This version looks better to me, generic/051 and generic/388 all test passed,
no panic or hang. More fstests regression tests didn't find critical issues.
(Just another ppc64le issue, looks like not related with this patch)

But I saw fd37b884003c ("io_uring/io-wq: don't clear PF_IO_WORKER on exit") has
been merged, so this might has to be another regression fix.

Thanks,
Zorro

>  
>  	kfree_rcu(worker, rcu);
>  	io_worker_ref_put(wq);
> 
> -- 
> Jens Axboe
>