Re: [PATCH 2/2] kernel: rerun task_work while freezing in get_signal()

[Oleg Nesterov <oleg@redhat.com>]

On 07/10, Tejun Heo wrote:
>
> Hello,
>
> On Wed, Jul 10, 2024 at 09:10:16PM +0200, Oleg Nesterov wrote:
> ...
> > If nothing else. CRIU needs to attach and make this task TASK_TRACED, right?
>
> Yeah, AFAIK, that's the only way to implement check-pointing for now.

OK,

> > And once the target task is traced, it won't react to task_work_add(TWA_SIGNAL).
>
> I don't know how task_work is being used but the requirement would be that
> if a cgroup is frozen, task_works shouldn't be making state changes which
> can't safely be replayed (e.g. by restarting the frozen syscalls).

Well, in theory task_work can do "anything".

Of course, it can't, say, restart a frozen syscall, task_work_run() just
executes the callbacks in kernel mode and returns.

> it'd be better to freeze them together.

And I tend to agree. simply beacase do_freezer_trap() (and more users of
clear_thread_flag(TIF_SIGPENDING) + schedule(TASK_INTERRUPTIBLE) pattern)
do not take TIF_NOTIFY_SIGNAL into account.

But how do you think this patch can make the things worse wrt CRIU ?

And let's even forget this patch which fixes the real problem.
How do you think the fact that the task sleeping in do_freezer_trap()
can react to TIF_NOTIFY_SIGNAL, call task_work_run(), and then sleep
in do_freezer_trap() again can make any difference in this sense?

> As this thing is kinda difficult to reason about,

Agreed,

> it'd probably be easier to just freeze them together if we can.

Agreed, but this needs some "generic" changes while Pavel needs a
simple and backportable workaround to suppress a real problem.

In short, I don't like this patch either, I just don't see a better
solution for now ;)

Thanks,

Oleg.