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

From: Peter Zijlstra <peterz@infradead.org>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Andi Kleen <andi@firstfloor.org>,
	Jan Stancek <jstancek@redhat.com>, Borislav Petkov <bp@alien8.de>
Subject: Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage!
Date: Mon, 21 Nov 2016 13:58:30 +0100	[thread overview]
Message-ID: <20161121125830.GE3092@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20161121093424.GA9814@krava>

On Mon, Nov 21, 2016 at 10:34:25AM +0100, Jiri Olsa wrote:

> > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > index 0888a879120f..d6c6aa80675f 100644
> > --- a/arch/x86/kernel/process.c
> > +++ b/arch/x86/kernel/process.c
> > @@ -357,7 +357,7 @@ static void amd_e400_idle(void)
> >  	if (!amd_e400_c1e_detected) {
> >  		u32 lo, hi;
> >  
> > -		rdmsr(MSR_K8_INT_PENDING_MSG, lo, hi);
> > +		RCU_NONIDLE(rdmsr(MSR_K8_INT_PENDING_MSG, lo, hi));
> >  
> >  		if (lo & K8_INTP_C1E_ACTIVE_MASK) {
> >  			amd_e400_c1e_detected = true;

OK, so while looking at this again, I don't like this ether :/

Problem with this one is that it always adds the RCU fiddling overhead,
even when we're not tracing.

I could do an rdmsr_notrace() for this one, dunno if its important.