Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage!

From: Steven Rostedt
Date: Mon Nov 21 2016 - 12:55:14 EST

On Mon, 21 Nov 2016 18:18:53 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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

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

-- Steve