[CRIU] [PATCHSET 0/2] PF_IO_WORKER signal tweaks

[Jens Axboe]

On 3/21/21 9:18 AM, Eric W. Biederman wrote:
> Jens Axboe <axboe at kernel.dk> writes:
> 
>> On 3/20/21 4:08 PM, Eric W. Biederman wrote:
>>>
>>> Added criu because I just realized that io_uring (which can open files
>>> from an io worker thread) looks to require some special handling for
>>> stopping and freezing processes.  If not in the SIGSTOP case in the
>>> related cgroup freezer case.
>>>
>>> Linus Torvalds <torvalds at linux-foundation.org> writes:
>>>
>>>> On Sat, Mar 20, 2021 at 10:51 AM Linus Torvalds
>>>> <torvalds at linux-foundation.org> wrote:
>>>>>
>>>>> Alternatively, make it not use
>>>>> CLONE_SIGHAND|CLONE_THREAD at all, but that would make it
>>>>> unnecessarily allocate its own signal state, so that's "cleaner" but
>>>>> not great either.
>>>>
>>>> Thinking some more about that, it would be problematic for things like
>>>> the resource counters too. They'd be much better shared.
>>>>
>>>> Not adding it to the thread list etc might be clever, but feels a bit too scary.
>>>>
>>>> So on the whole I think Jens' minor patches to just not have IO helper
>>>> threads accept signals are probably the right thing to do.
>>>
>>> The way I see it we have two options:
>>>
>>> 1) Don't ask PF_IO_WORKERs to stop do_signal_stop and in
>>>    task_join_group_stop.
>>>
>>>    The easiest comprehensive implementation looks like just
>>>    updating task_set_jobctl_pending to treat PF_IO_WORKER
>>>    as it treats PF_EXITING.
>>>
>>> 2) Have the main loop of the kernel thread test for JOBCTL_STOP_PENDING
>>>    and call into do_signal_stop.
>>>
>>> It is a wee bit trickier to modify the io_workers to stop, but it does
>>> not look prohibitively difficult.
>>>
>>> All of the work performed by the io worker is work scheduled via
>>> io_uring by the process being stopped.
>>>
>>> - Is the amount of work performed by the io worker thread sufficiently
>>>   negligible that we don't care?
>>>
>>> - Or is the amount of work performed by the io worker so great that it
>>>   becomes a way for an errant process to escape SIGSTOP?
>>>
>>> As the code is all intermingled with the cgroup_freezer.  I am also
>>> wondering creating checkpoints needs additional stopping guarantees.
>>
>> The work done is the same a syscall, basically. So it could be long
>> running and essentially not doing anything (eg read from a socket, no
>> data is there), or it's pretty short lived (eg read from a file, just
>> waiting on DMA).
>>
>> This is outside of my domain of expertise, which is exactly why I added
>> you and Linus to make some calls on what the best approach here would
>> be. My two patches obviously go route #1 in terms of STOP. And fwiw,
>> I tested this:
>>
>>> To solve the issue that SIGSTOP is simply broken right now I am totally
>>> fine with something like:
>>>
>>> diff --git a/kernel/signal.c b/kernel/signal.c
>>> index ba4d1ef39a9e..cb9acdfb32fa 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)
>>
>> and can confirm it works fine for me with 2/2 reverted and this applied
>> instead.
>>
>>> Which just keeps from creating unstoppable processes today.  I am just
>>> not convinced that is what we want as a long term solution.
>>
>> How about we go with either my 2/2 or yours above to at least ensure we
>> don't leave workers looping as schedule() is a nop with sigpending? If
>> there's a longer timeline concern that "evading" SIGSTOP is a concern, I
>> have absolutely no qualms with making the IO threads participate. But
>> since it seems conceptually simple but with potentially lurking minor
>> issues, probably not the ideal approach for right now.
> 
> 
> Here is the signoff for mine.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
> 
> Yours misses the joining of group stop during fork.  So we better use
> mine.

I've updated it and attributed it to you, here is is:

https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-5.12&id=4db4b1a0d1779dc159f7b87feb97030ec0b12597

> As far as I can see that fixes the outstanding bugs.

Great!

> Jens can you make a proper patch out of it and send it to Linus for
> -rc4?  I unfortunately have other commitments and this is all I can do
> for today.

Will do - I'm going to sanity run the current branch and do a followup
pull request for Linus once I've verified everything is still sane.

-- 
Jens Axboe