linux-kernel - Re: [PATCH -v5 12/17] x86/kprobes: Fix ordering

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Date:   Thu, 14 Nov 2019 16:58:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     paulmck <paulmck@...nel.org>, x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        bristot <bristot@...hat.com>, jbaron <jbaron@...mai.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Nadav Amit <namit@...are.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jessica Yu <jeyu@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH -v5 12/17] x86/kprobes: Fix ordering

On Thu, Nov 14, 2019 at 04:42:55PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 14, 2019 at 10:30:01AM -0500, Mathieu Desnoyers wrote:
> > ----- On Nov 14, 2019, at 10:28 AM, Peter Zijlstra peterz@...radead.org wrote:
> 
> > > I don't think that is needed. As per the patch under discussion, we
> > > unconditionally need that IPI-sync (even for !optimized) but we only
> > > need the synchonize_rcu_tasks() thing for optimized kprobes.
> > > 
> > > Also, they really do two different things. Lets not tie them together.
> > 
> > I'm fine with this approach, I just thought it would be good to consider
> > the alternative.
> 
> Fair enough; I also just remembered we use synchronize_rcu_tasks() in
> scenarios where we don't need to IPI-sync, for instrance when freeing
> ftrace trampolines. There we just want to make sure nothing is still
> preempted inside the trampoline when we free it -- which would be BAD
> :-)

One more thing. I have a TODO item for making text_poke_sync() more
complicated.

Specifically we don't have to IPI CPUs that are in NOHZ_FULL userspace
context, provided we can make the kernel entry perform the
core-serialize thing.

It's not far up the TODO list though, but I figure I'd mention it just
in case other people fancy having a go.