On Thu 2020-10-01 13:13:07, Miroslav Benes wrote:

On Wed, 30 Sep 2020, Evgenii Shatokhin wrote:

quoted

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.
 

quoted

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