linux-kernel - Re: [PATCH AUTOSEL 5.11 43/44] signal: don't allow STOP on PF_IO

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Date:   Thu, 25 Mar 2021 11:00:45 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Stefan Metzmacher <metze@...ba.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        io-uring <io-uring@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH AUTOSEL 5.11 43/44] signal: don't allow STOP on
 PF_IO_WORKER threads

On Thu, Mar 25, 2021 at 08:02:11AM -0600, Jens Axboe wrote:
>On 3/25/21 7:56 AM, Stefan Metzmacher wrote:
>> Am 25.03.21 um 14:38 schrieb Jens Axboe:
>>> On 3/25/21 6:11 AM, Stefan Metzmacher wrote:
>>>>
>>>> Am 25.03.21 um 13:04 schrieb Eric W. Biederman:
>>>>> Stefan Metzmacher <metze@...ba.org> writes:
>>>>>
>>>>>> Am 25.03.21 um 12:24 schrieb Sasha Levin:
>>>>>>> From: "Eric W. Biederman" <ebiederm@...ssion.com>
>>>>>>>
>>>>>>> [ Upstream commit 4db4b1a0d1779dc159f7b87feb97030ec0b12597 ]
>>>>>>>
>>>>>>> Just like we don't allow normal signals to IO threads, don't deliver a
>>>>>>> STOP to a task that has PF_IO_WORKER set. The IO threads don't take
>>>>>>> signals in general, and have no means of flushing out a stop either.
>>>>>>>
>>>>>>> Longer term, we may want to look into allowing stop of these threads,
>>>>>>> as it relates to eg process freezing. For now, this prevents a spin
>>>>>>> issue if a SIGSTOP is delivered to the parent task.
>>>>>>>
>>>>>>> Reported-by: Stefan Metzmacher <metze@...ba.org>
>>>>>>> Signed-off-by: Jens Axboe <axboe@...nel.dk>
>>>>>>> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>>>>>>> Signed-off-by: Sasha Levin <sashal@...nel.org>
>>>>>>> ---
>>>>>>>  kernel/signal.c | 3 ++-
>>>>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/kernel/signal.c b/kernel/signal.c
>>>>>>> index 55526b941011..00a3840f6037 100644
>>>>>>> --- a/kernel/signal.c
>>>>>>> +++ b/kernel/signal.c
>>>>>>> @@ -288,7 +288,8 @@ bool task_set_jobctl_pending(struct task_struct *task, unsigned long mask)
>>>>>>>  			JOBCTL_STOP_SIGMASK | JOBCTL_TRAPPING));
>>>>>>>  	BUG_ON((mask & JOBCTL_TRAPPING) && !(mask & JOBCTL_PENDING_MASK));
>>>>>>>
>>>>>>> -	if (unlikely(fatal_signal_pending(task) || (task->flags & PF_EXITING)))
>>>>>>> +	if (unlikely(fatal_signal_pending(task) ||
>>>>>>> +		     (task->flags & (PF_EXITING | PF_IO_WORKER))))
>>>>>>>  		return false;
>>>>>>>
>>>>>>>  	if (mask & JOBCTL_STOP_SIGMASK)
>>>>>>>
>>>>>>
>>>>>> Again, why is this proposed for 5.11 and 5.10 already?
>>>>>
>>>>> Has the bit about the io worker kthreads been backported?
>>>>> If so this isn't horrible.  If not this is nonsense.
>>>
>>> No not yet - my plan is to do that, but not until we're 100% satisfied
>>> with it.
>>
>> Do you understand why the patches where autoselected for 5.11 and 5.10?
>
>As far as I know, selections like these (AUTOSEL) are done by some bot
>that uses whatever criteria to see if they should be applied for earlier
>revisions. I'm sure Sasha can expand on that :-)

Right, it's just heuristics that help catch commits that don't have a
stable tag but should have one.

>Hence it's reasonable to expect that sometimes it'll pick patches that
>should not go into stable, at least not just yet. It's important to
>understand that this message is just a notice that it's queued up for
>stable -rc, not that it's _in_ stable just yet. There's time to object.

Right, it's even more than that: this mail (tagged with "AUTOSEL") is a
notification that happens at least a week before the patch will go in
the stable queue.

If you think any AUTOSEL patches don't need to be backported, it's
usually enough to just quickly nack them.

-- 
Thanks,
Sasha