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

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Date:   Wed, 15 Nov 2017 19:16:42 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     jeyu@...nel.org, jikos@...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, Nov 15, 2017 at 02:50:12PM +0100, 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

For the series:

Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>

-- 
Josh