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

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Date:   Thu, 14 Nov 2019 16:28:29 +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>
Subject: Re: [PATCH -v5 12/17] x86/kprobes: Fix ordering

On Thu, Nov 14, 2019 at 10:22:24AM -0500, Mathieu Desnoyers wrote:

> >> > So what we do, after enabling the regular kprobe, is call
> >> > synchronize_rcu_tasks() to wait for each task to have passed through
> >> > schedule(). That guarantees no task is preempted inside the kprobe
> >> > shadow (when it triggers it ensures it resumes execution at an
> >> > instruction boundary further than 5 bytes away).
> >> 
> >> Indeed, given that synchronize_rcu_tasks() awaits for voluntary context
> >> switches (or user-space execution), it guarantees that no task was preempted
> >> within the kprobe shadow.
> >> 
> >> Considering that synchronize_rcu_tasks() is meant only for code rewriting,
> >> I wonder if it would make sense to include the core serializing guarantees
> >> within this RCU API ?
> > 
> > As in have synchronize_rcu_tasks() do the IPI-sync love before doing
> > the current wait-for-voluntary-context-switch work?
> 
> This is what I have in mind, yes, based on the assumption that the only
> intended use-case for synchronize_rcu_tasks() is code patching.

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.