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From: Petr Mladek <pmladek@suse.com>
To: Miroslav Benes <mbenes@suse.cz>
Cc: Evgenii Shatokhin <eshatokhin@virtuozzo.com>,
	live-patching@vger.kernel.org, nstange@suse.de
Subject: Re: Patching kthread functions
Date: Thu, 1 Oct 2020 16:46:26 +0200	[thread overview]
Message-ID: <20201001144625.GE17717@alley> (raw)
In-Reply-To: <alpine.LSU.2.21.2010011300450.6689@pobox.suse.cz>

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

  parent reply	other threads:[~2020-10-01 14:46 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-30 15:44 Patching kthread functions Evgenii Shatokhin
2020-10-01 11:13 ` Miroslav Benes
2020-10-01 12:43   ` Nicolai Stange
2020-10-01 13:18     ` Evgenii Shatokhin
2020-10-01 13:12   ` Evgenii Shatokhin
2020-10-02 11:53     ` Miroslav Benes
2020-10-02 12:52       ` Evgenii Shatokhin
2020-10-02 13:06         ` Miroslav Benes
2020-10-01 14:46   ` Petr Mladek [this message]
2020-10-01 16:34     ` Evgenii Shatokhin

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