linux-kernel - Re: [PATCH v4 0/2] livepatch: Introduce signal and force sysfs attributes

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Date:   Mon, 4 Dec 2017 22:37:24 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Miroslav Benes <mbenes@...e.cz>
cc:     jpoimboe@...hat.com, jeyu@...nel.org, pmladek@...e.com,
        lpechacek@...e.cz, pavel@....cz, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Oleg Nesterov <oleg@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 0/2] livepatch: Introduce signal and force sysfs
 attributes

On Wed, 15 Nov 2017, Miroslav Benes wrote:

> Currently, livepatch gradually migrate the system from an unpatched to a
> patched state (or vice versa). Each task drops its TIF_PATCH_PENDING
> itself when crossing the kernel/user space boundary or it is cleared
> using the stack checking approach. If there is a task which sleeps on a
> patched function, the whole transition can get stuck indefinitely.
> 
> Livepatch has means which can be used in these cases. The transition can
> be cancelled and/or immediate flag may be used for the live patch. On
> the other hand it might be useful to poke the system a little bit and
> help the transition to finish by doing so.
> 
> That is what the fake signal can be used for. A task sleeping/waiting in
> the kernel gets TIF_SIGPENDING set, it handles it and during that its
> TIF_PATCH_PENDING is cleared. Kthreads are only woken up, they do not
> handle signals suitably.
> 
> Still, there are cases which neither fake signal can solve. A task can
> sleep uninterruptedly without reacting to signals at all. Even then, it
> may be safe to clear the task's TIF_PATCH_PENDING. As a last resort,
> admin may force such clearing for all tasks in the system with this
> patch set.
> 
> We use the fake signal in SLES for a long time. Moreover, we don't have
> a stack checking there, so we rely on the fake signal a lot. We send it
> automatically and periodically.
> 
> Petr, I did not add you Reviewed-by tags intentionally because of the changes.
> 
> Changes from v3:
> - only TASK_INTERRUPTIBLE kthreads are woken up - Petr
> - documentation - Pavel
> - function naming and sysfs fix - Josh
> 
> Changes from v2:
> - two sysfs attributes instead of one - Petr, Josh
> - better documentation about force usage - Pavel
> - small changes here and there
> 
> Changes from v1:
> - better wording, typos, comments, documentation - Libor, Josh
> - symbolic names in sysfs instead of numbers - Libor
> - exit_to_usermode_loop(), call klp_update_patch_state() before do_signal() - Oleg
> - better names - Josh
> - mutex and WARN_ON_ONCE not added to klp_force_transitions() - Petr, Josh
> - handle idle tasks in klp_force_transitions() too - Josh
> 
> Miroslav Benes (2):
>   livepatch: send a fake signal to all blocking tasks
>   livepatch: force transition to finish
> 
>  Documentation/ABI/testing/sysfs-kernel-livepatch | 25 +++++++++
>  Documentation/livepatch/livepatch.txt            | 22 +++++++-
>  arch/powerpc/kernel/signal.c                     |  6 +--
>  arch/x86/entry/common.c                          |  6 +--
>  kernel/livepatch/core.c                          | 60 +++++++++++++++++++++
>  kernel/livepatch/transition.c                    | 66 ++++++++++++++++++++++++
>  kernel/livepatch/transition.h                    |  2 +
>  kernel/signal.c                                  |  4 +-

This is now queued in livepatching.git#for-4.16/signal-sysfs-force. 
Thanks,

-- 
Jiri Kosina
SUSE Labs