linux-kernel - Re: [PATCH v5 13/15] livepatch: change to a per-task consistency model

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Message-ID: <20180125102414.wu5od5ye5nmx2ckl@pathway.suse.cz>
Date:   Thu, 25 Jan 2018 11:24:14 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, Jessica Yu <jeyu@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>, x86@...nel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        Vojtech Pavlik <vojtech@...e.com>, Jiri Slaby <jslaby@...e.cz>,
        Chris J Arges <chris.j.arges@...onical.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        Balbir Singh <bsingharora@...il.com>
Subject: Re: [PATCH v5 13/15] livepatch: change to a per-task consistency
 model

On Thu 2018-01-25 10:04:44, Peter Zijlstra wrote:
> On Mon, Feb 13, 2017 at 07:42:40PM -0600, Josh Poimboeuf wrote:
> > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> > index 6a4bae0..a8b3f1a 100644
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -9,6 +9,7 @@
> >  #include <linux/mm.h>
> >  #include <linux/stackprotector.h>
> >  #include <linux/suspend.h>
> > +#include <linux/livepatch.h>
> >  
> >  #include <asm/tlb.h>
> >  
> > @@ -264,6 +265,9 @@ static void do_idle(void)
> >  
> >  	sched_ttwu_pending();
> >  	schedule_preempt_disabled();
> > +
> > +	if (unlikely(klp_patch_pending(current)))
> > +		klp_update_patch_state(current);
> >  }
> 
> Can someone explain this one? This is a very weird place to add things.
> What was the expectation?

AFAIK, it was the least ugly and minimalist solution that we came with.

The tasks are migrated to the new patch when neither of the patched
functions is on the stack. The stack can be checked safely only when
the task is not running. It might be very hard to catch the idle
task on a such a place if we patch something that is used there.

If the idle task is scheduled, you would need to create some fake
load on the system, try to migrate the idle task, stop the fake load
on the CPU.

The above code makes the idle task to migrate itself on a sane place.
You just need to schedule some minimalist job on the CPU. The idle
task will do one loop, gets migrated, and might be scheduled again
immediately.

Best Regards,
Petr