Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH 2/8] kernel: unmask SIGSTOP for IO threads
To:     Stefan Metzmacher <metze@samba.org>,
        Oleg Nesterov <oleg@redhat.com>
Cc:     io-uring@vger.kernel.org, torvalds@linux-foundation.org,
        ebiederm@xmission.com, linux-kernel@vger.kernel.org
References: <20210326003928.978750-1-axboe@kernel.dk>
 <20210326003928.978750-3-axboe@kernel.dk> <20210326134840.GA1290@redhat.com>
 <a179ad33-5656-b644-0d92-e74a6bd26cc8@kernel.dk>
 <8f2a4b48-77c9-393f-5194-100ed63c05fc@samba.org>
 <58f67a8b-166e-f19c-ccac-157153e4f17c@kernel.dk>
 <c61fc5eb-c997-738b-1a60-5e3db2754f49@samba.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <857d1456-2258-f798-5927-ca4d82f10d50@kernel.dk>
Date:   Fri, 26 Mar 2021 12:59:01 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.10.0
MIME-Version: 1.0
In-Reply-To: <c61fc5eb-c997-738b-1a60-5e3db2754f49@samba.org>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

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@kernel.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