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

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Date:   Mon, 21 Nov 2016 15:37:16 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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>,
        Andi Kleen <andi@...stfloor.org>,
        Jan Stancek <jstancek@...hat.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage!

On Mon, Nov 21, 2016 at 09:15:43AM -0500, Steven Rostedt wrote:
> On Mon, 21 Nov 2016 13:58:30 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > 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.
> 
> But that would neglect the point of tracing rdmsr. What about:
> 
> 	/* tracepoints require RCU enabled */
> 	if (trace_read_msr_enabled())
> 		RCU_NONIDLE(rdmsr(MSR_K8_INT_PENDING_MSG, lo, hi));
> 	else
> 		rdmsr(MSR_K8_INT_PENDING_MSG, lo, hi);

Yeah, but this one does a printk() when it hits the contidion it checks
for, so not tracing it would be fine I think.

Also, Boris, why do we need to redo that rdmsr until we see that bit
set? Can't we simply do the rdmsr once and then be done with it?