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

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Date:   Tue, 3 Oct 2017 19:57:22 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>, mingo@...hat.com, x86@...nel.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


Sorry for the late reply. Coming back from Kernel Recipes, I fell way
behind in email.

On Fri, 29 Sep 2017 00:29:38 -0700
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> > > +	 * are done. Because optprobe may modify multiple instructions,
> > > +	 * there is a chance that the Nth instruction is interrupted. In that
> > > +	 * case, running interrupt can return to the Nth byte of jump
> > > +	 * instruction. This can be avoided by waiting for returning of
> > > +	 * such interrupts, since (until here) the first byte of the optimized
> > > +	 * probe is already replaced with normal kprobe (sw breakpoint) and
> > > +	 * all threads which reach to the probed address will hit it and
> > > +	 * bypass the copied instructions (instead of executing the original.)
> > > +	 * With CONFIG_PREEMPT, such interrupts can be preepmted. To wait
> > > +	 * for such thread, we will use synchronize_rcu_tasks() which ensures
> > > +	 * all preeempted tasks are scheduled normally (not preempted).
> > > +	 * So we can ensure there is no threads preempted at probed address.  
> > 
> > What? Interrupts cannot be preempted.  
> 
> Steve, could you correct me if I'm wrong. I thought if the kernel is
> compiled with CONFIG_PREEMPT=y, even in the kernel, it can be preempted
> suddenly. It means timer interrupt occurs at kernel path and it yield
> to new task (=preempt.) Do I miss something?

The above sounds correct. I believe Ingo was pointing out the line that
states "With CONFIG_PREEMPT, such interrupts can be preempted", which
is not true. I think you meant that interrupts can preempt the kernel
and cause it to schedule out. The line above sounds like you meant the
interrupt was preempted, which can't happen.

-- Steve