Re: [PATCHSET 0/2] PF_IO_WORKER signal tweaks

From: Jens Axboe
Date: Sun Mar 21 2021 - 11:43:56 EST

On 3/21/21 9:18 AM, Eric W. Biederman wrote:
> Jens Axboe <axboe@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx> writes:
>>>> On Sat, Mar 20, 2021 at 10:51 AM Linus Torvalds
>>>> <torvalds@xxxxxxxxxxxxxxxxxxxx> 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)
>>> - 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@xxxxxxxxxxxx>
> 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:

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


> 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