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Date:   Tue, 11 Jun 2019 12:11:12 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Masami Hiramatsu <mhiramat@kernel.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Nadav Amit <namit@vmware.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate
 instructions
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On Tue, 11 Jun 2019 08:54:23 -0700
Andy Lutomirski <luto@amacapital.net> wrote:


> How does that help?  If RIP == x+2 and you want to put a 5-byte jump
> at address x, no amount of 0xcc is going to change the fact that RIP
> is in the middle of the jump.
> 
> Live patching can handle this by detecting this condition on each
> CPU, but performance won’t be great.  Maybe some synchronize_sched
> trickery could help.

We have synchronize_rcu_tasks() which return after all tasks have
either entered user space or did a voluntary schedule (was not
preempted). Or have not run (still in a sleeping state).

That way we guarantee that all tasks are no longer on any trampoline
or code paths that do not call schedule. I use this to free dynamically
allocated trampolines used by ftrace. And kprobes uses this too for its
own trampolines.

-- Steve