linux-kernel - Re: [PATCH] io_uring/io-wq: don't clear PF_IO

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Message-ID: <18f7cc1e-5e0c-def9-4d86-d7971df17f54@kernel.dk>
Date:   Wed, 14 Jun 2023 20:23:28 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Zorro Lang <zlang@...hat.com>
Cc:     io-uring <io-uring@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH] io_uring/io-wq: don't clear PF_IO_WORKER on exit

On 6/14/23 8:22?PM, Zorro Lang wrote:
> 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@...hat.com>
>>>> Reported-by: Dave Chinner <david@...morbit.com>
>>>> Reported-by: Linus Torvalds <torvalds@...ux-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@...nel.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)

Good, thanks for testing!

> 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.

Yep, this fix will go out tomorrow/friday.

-- 
Jens Axboe