linux-kernel - Re: [PATCH] Documentation/livepatch: remove the limitation for schedule() patching

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Date:   Fri, 6 Jan 2017 16:10:50 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Petr Mladek <pmladek@...e.com>
cc:     jpoimboe@...hat.com, jeyu@...hat.com, jikos@...nel.org,
        corbet@....net, live-patching@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Documentation/livepatch: remove the limitation for
 schedule() patching

On Fri, 6 Jan 2017, Petr Mladek wrote:

> On Fri 2017-01-06 15:00:45, Miroslav Benes wrote:
> > The Limitations section of the documentation describes the impossibility
> > to livepatch anything that is inlined to __schedule() function. This had
> > been true till 4.9 kernel came. Thanks to commit 0100301bfdf5
> > ("sched/x86: Rewrite the switch_to() code") from Brian Gerst there is
> > __switch_to_asm function now (implemented in assembly) called properly
> > from context_switch(). RIP is thus saved on the stack and a task would
> > return to proper version of __schedule() et al. functions.
> > 
> > Of course __switch_to_asm() is not patchable for the reason described in
> > the section. But there is no __fentry__ call and I cannot imagine a
> > reason to do it anyway.
> > 
> > Therefore, remove the paragraphs from the section.
> > 
> > Signed-off-by: Miroslav Benes <mbenes@...e.cz>
> 
> It is great to get a feature for free ;-)
> 
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> 
> Best Regards,
> Petr
> 
> ---
> > FWIW, I also tested this to be sure on top of the consistency model
> > patch set. I patched schedule() function which calls __schedule() (it is
> > impossible to patch it directly due to notrace attribute). It works well
> > except...
> > 
> > 1. the patching process does not finish, because many tasks sleep in
> > schedule. STOP/CONT signal does not help. I'll investigate.
> 
> Are these userspace processes or kthreads? Kthreads would cause
> problems because they do not handle signals.

Userspace processes, but I take it back. Stupid typo in my script. It 
works as expected. Kthreads sleeping in schedule() are of course there and 
a signal does not help.

Miroslav