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

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Message-Id: <20170929074436.f447187d99e5e78008b15234@kernel.org>
Date:   Fri, 29 Sep 2017 07:44:36 -0700
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Ingo Molnar <mingo@...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

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!

Thank you,

> 
> or something else?
> 
> Thanks,
> 
> 	Ingo


-- 
Masami Hiramatsu <mhiramat@...nel.org>