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From:   Miroslav Benes <mbenes@suse.cz>
To:     jpoimboe@redhat.com, jeyu@kernel.org, jikos@kernel.org
Cc:     pmladek@suse.com, lpechacek@suse.cz, pavel@ucw.cz,
        live-patching@vger.kernel.org, linux-kernel@vger.kernel.org,
        Miroslav Benes <mbenes@suse.cz>,
        Andy Lutomirski <luto@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Oleg Nesterov <oleg@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: [PATCH v4 0/2] livepatch: Introduce signal and force sysfs attributes
Date:   Wed, 15 Nov 2017 14:50:12 +0100
Message-Id: <20171115135014.20594-1-mbenes@suse.cz>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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 +-
 8 files changed, 182 insertions(+), 9 deletions(-)

-- 
2.15.0


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