Re: Patching kthread functions

[Petr Mladek <pmladek@suse.com>]

On Thu 2020-10-01 13:13:07, Miroslav Benes wrote:
> On Wed, 30 Sep 2020, Evgenii Shatokhin wrote:
> 
> > Hi,
> > 
> > I wonder, can livepatch from the current mainline kernel patch the main
> > functions of kthreads, which are running or sleeping constantly? Are there any
> > best practices here?
> 
> No. It is a "known" limitation, "" because we discussed it a couple of 
> times (at least with Petr), but it is not documented :(
> 
> I wonder if it is really an issue practically. I haven't met a case 
> yet when we wanted to patch such thing. But yes, you're correct, it is not 
> possible.
>  
> > I mean, suppose we have a function which runs in a kthread (passed to
> > kthread_create()) and is organized like this:
> > 
> > while (!kthread_should_stop()) {
> >   ...
> >   DEFINE_WAIT(_wait);
> >   for (;;) {
> >     prepare_to_wait(waitq, &_wait, TASK_INTERRUPTIBLE);
> >     if (we_have_requests_to_process || kthread_should_stop())
> >       break;
> >     schedule();
> >   }
> >   finish_wait(waitq, &_wait);
> >   ...
> >   if (we_have_requests_to_process)
> >     process_one_request();
> >   ...
> > }

Crazy hack would be to patch only process_one_request() the following way:

1. Put the fixed main loop into the new process_one_request() function.

2. Put the original process_one_request() code into another function,
   e.g. do_process_one_request_for_real() and call it from the
   fixed loop.

Does it make any sense or should I provide some code?

Be aware that such patch could not get reverted because it would never
leave the new main loop.

Best Regards,
Petr