Re: [PATCH AUTOSEL 5.11 43/44] signal: don't allow STOP on PF_IO_WORKER threads

Stefan Metzmacher
2021-03-25T12:12:23Z

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.

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.

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

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.

metze