linux-kernel - Re: [BUG] msr-trace.h:42 suspicious rcu_dereference

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Date:   Mon, 21 Nov 2016 12:55:01 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andi Kleen <andi@...stfloor.org>, Jiri Olsa <jolsa@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Jan Stancek <jstancek@...hat.com>
Subject: Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage!

On Mon, 21 Nov 2016 18:18:53 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> Its not ftrace as such though, its RCU, ftrace simply uses RCU to avoid
> locking, as one does.

Just to be clear, as ftrace in the kernel mostly represents function
tracing, which doesn't use RCU. This is a tracepoint feature.

> 
> Biggest objection would be that the rcu_irq_enter_irqson() thing does
> POPF and rcu_irq_exit_irqson() does again. So wrapping every tracepoint
> with that is quite a few cycles.

Agree. Even though this ends up being a whack-a-mole(TM) fix, I'm not
concerned enough to put a heavy weight rcu idle code in for all
tracepoints.

Although, what about a percpu flag that can be checked in the
tracepoint code to see if it should enable RCU or not?

Hmm, I wonder if "rcu_is_watching()" is light enough to have in all
tracepoints?

-- Steve