linux-kernel - Re: [PATCH 2/8] kernel: unmask SIGSTOP for IO threads

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Date:   Fri, 26 Mar 2021 09:29:32 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Stefan Metzmacher <metze@...ba.org>,
        Oleg Nesterov <oleg@...hat.com>
Cc:     io-uring@...r.kernel.org, torvalds@...ux-foundation.org,
        ebiederm@...ssion.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/8] kernel: unmask SIGSTOP for IO threads

On 3/26/21 9:23 AM, Stefan Metzmacher wrote:
> Am 26.03.21 um 16:01 schrieb Jens Axboe:
>> On 3/26/21 7:48 AM, Oleg Nesterov wrote:
>>> Jens, sorry, I got lost :/
>>
>> Let's bring you back in :-)
>>
>>> On 03/25, Jens Axboe wrote:
>>>>
>>>> With IO threads accepting signals, including SIGSTOP,
>>>
>>> where can I find this change? Looks like I wasn't cc'ed...
>>
>> It's this very series.
>>
>>>> unmask the
>>>> SIGSTOP signal from the default blocked mask.
>>>>
>>>> Signed-off-by: Jens Axboe <axboe@...nel.dk>
>>>> ---
>>>>  kernel/fork.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>>> index d3171e8e88e5..d5a40552910f 100644
>>>> --- a/kernel/fork.c
>>>> +++ b/kernel/fork.c
>>>> @@ -2435,7 +2435,7 @@ struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node)
>>>>  	tsk = copy_process(NULL, 0, node, &args);
>>>>  	if (!IS_ERR(tsk)) {
>>>>  		sigfillset(&tsk->blocked);
>>>> -		sigdelsetmask(&tsk->blocked, sigmask(SIGKILL));
>>>> +		sigdelsetmask(&tsk->blocked, sigmask(SIGKILL)|sigmask(SIGSTOP));
>>>
>>> siginitsetinv(blocked, sigmask(SIGKILL)|sigmask(SIGSTOP)) but this is minor.
>>
>> Ah thanks.
>>
>>> To remind, either way this is racy and can't really help.
>>>
>>> And if "IO threads accepting signals" then I don't understand why. Sorry,
>>> I must have missed something.
>>
>> I do think the above is a no-op at this point, and we can probably just
>> kill it. Let me double check, hopefully we can just remove this blocked
>> part.
> 
> Is this really correct to drop in your "kernel: stop masking signals in create_io_thread()"
> commit?
> 
> I don't assume signals wanted by userspace should potentially handled in an io_thread...
> e.g. things set with fcntl(fd, F_SETSIG,) used together with F_SETLEASE?

I guess we do actually need it, if we're not fiddling with
wants_signal() for them. To quell Oleg's concerns, we can just move it
to post dup_task_struct(), that should eliminate any race concerns
there.

-- 
Jens Axboe