linux-kernel - [PATCH v3 0/2] livepatch: Introduce signal and force sysfs attributes

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Date:   Tue, 31 Oct 2017 12:48:51 +0100
From:   Miroslav Benes <mbenes@...e.cz>
To:     jpoimboe@...hat.com, jeyu@...nel.org, jikos@...nel.org
Cc:     pmladek@...e.com, lpechacek@...e.cz, pavel@....cz,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Miroslav Benes <mbenes@...e.cz>,
        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: [PATCH v3 0/2] livepatch: Introduce signal and force sysfs attributes

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.

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 process to finish

 Documentation/ABI/testing/sysfs-kernel-livepatch | 19 +++++++
 Documentation/livepatch/livepatch.txt            | 21 ++++++++
 arch/powerpc/kernel/signal.c                     |  6 +--
 arch/x86/entry/common.c                          |  6 +--
 kernel/livepatch/core.c                          | 54 ++++++++++++++++++++
 kernel/livepatch/transition.c                    | 65 ++++++++++++++++++++++++
 kernel/livepatch/transition.h                    |  2 +
 kernel/signal.c                                  |  4 +-
 8 files changed, 170 insertions(+), 7 deletions(-)

-- 
2.14.3