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Subject: Re: [PATCH AUTOSEL 5.11 43/44] signal: don't allow STOP on
 PF_IO_WORKER threads
To:     Stefan Metzmacher <metze@samba.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Sasha Levin <sashal@kernel.org>, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, io-uring <io-uring@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
References: <20210325112459.1926846-1-sashal@kernel.org>
 <20210325112459.1926846-43-sashal@kernel.org>
 <f4c932b4-b787-651e-dd9f-584b386acddb@samba.org>
 <m1r1k34ey1.fsf@fess.ebiederm.org>
 <41589c56-9219-3ec2-55b3-3f010752ac7b@samba.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <2b2a9701-cbe0-4538-ed3b-6917b85bebf8@kernel.dk>
Date:   Thu, 25 Mar 2021 07:38:51 -0600
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On 3/25/21 6:11 AM, Stefan Metzmacher wrote:
> 
> Am 25.03.21 um 13:04 schrieb Eric W. Biederman:
>> Stefan Metzmacher <metze@samba.org> writes:
>>
>>> Am 25.03.21 um 12:24 schrieb Sasha Levin:
>>>> From: "Eric W. Biederman" <ebiederm@xmission.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@samba.org>
>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>>>> Signed-off-by: Sasha Levin <sashal@kernel.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.

> I don't know, I hope not...
> 
> But I just tested v5.12-rc4 and attaching to
> an application with iothreads with gdb is still not possible,
> it still loops forever trying to attach to the iothreads.

I do see the looping, gdb apparently doesn't give up when it gets
-EPERM trying to attach to the threads. Which isn't really a kernel
thing, but:

> And I tested 'kill -9 $pidofiothread', and it feezed the whole
> machine...

that sounds very strange, I haven't seen anything like that running
the exact same scenario.

> So there's still work to do in order to get 5.12 stable.
> 
> I'm short on time currently, but I hope to send more details soon.

Thanks! I'll play with it this morning and see if I can provoke
something odd related to STOP/attach.

-- 
Jens Axboe