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

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Date:   Fri, 26 Mar 2021 12:59:01 -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 12:01 PM, Stefan Metzmacher wrote:
> Am 26.03.21 um 16:29 schrieb Jens Axboe:
>> 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.
> 
> If that one is racy, don' we better also want this one?
> https://lore.kernel.org/io-uring/438b738c1e4827a7fdfe43087da88bbe17eedc72.1616197787.git.metze@samba.org/T/#u
> 
> And clear tsk->pf_io_worker ?

Definitely prudent. I'll get round 2 queued up shortly.

-- 
Jens Axboe