linux-kernel - Re: [PATCH -tip v3 7/7] kprobes: Use synchronize_rcu_tasks() for optprobe with CONFIG

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Message-ID: <20170929174527.ajbzh5jljaffh57x@gmail.com>
Date:   Fri, 29 Sep 2017 19:45:28 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     mingo@...hat.com, x86@...nel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Alexei Starovoitov <ast@...com>
Subject: Re: [PATCH -tip v3 7/7] kprobes: Use synchronize_rcu_tasks() for
 optprobe with CONFIG_PREEMPT


* Masami Hiramatsu <mhiramat@...nel.org> wrote:

> On Fri, 29 Sep 2017 09:37:55 +0200
> Ingo Molnar <mingo@...nel.org> wrote:
> 
> > 
> > * Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > 
> > > On Thu, 28 Sep 2017 09:22:20 +0200
> > > Ingo Molnar <mingo@...nel.org> wrote:
> > > 
> > > > 
> > > > * Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > > > 
> > > > > To enable jump optimized probe with CONFIG_PREEMPT, use
> > > > > synchronize_rcu_tasks() to wait for all tasks preempted
> > > > > on trampoline code back on track.
> > > > 
> > > > This sentence does not parse. It's missing a verb, but I'm not sure.
> > > 
> > > Hmm, how about this?
> > > 
> > > Use synchthnize_rcu_tasks() to wait for all tasks preempted
> > > on trampoline code back on track so that jump optimized probe
> > > can be enabled with CONFIG_PREEMPT.
> > 
> > What's "synchthnize"? ...
> 
> Oops, it's my typo. my XPS touch pad is really unstable...
> 
> > 
> > More seriously, I still don't understand it. What is 'back on track'?
> > 
> > Do you mean to say:
> > 
> >    We want to wait for all potentially preempted kprobes trampoline execution to 
> >    have completed. This guarantees that any freed trampoline memory is not in use
> >    by any task in the system anymore. synchronize_rcu_tasks() gives such a
> >    guarantee, so use it.
> 
> Exactly, this is correct!

Ok, great - please re-send the remaining kprobes patches that I have not applied 
yet - I'll read through the changelogs and fix any bits that might still be 
unclear.

Thanks,

	Ingo