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

From Peter Zijlstra <peterz@infradead.org>
Newsgroups linux.kernel
Subject Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage!
Date 2016-11-21 15:40 +0100
Message-ID <sFXVf-2nj-29@gated-at.bofh.it> (permalink)
References <sFL7H-2pc-3@gated-at.bofh.it> <sFT5f-7LV-3@gated-at.bofh.it> <sFTeW-7P2-9@gated-at.bofh.it> <sFWmz-1in-13@gated-at.bofh.it> <sFXLz-2km-9@gated-at.bofh.it>
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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@infradead.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?

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[BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Jiri Olsa <jolsa@redhat.com> - 2016-11-21 02:00 +0100
  Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-11-21 10:10 +0100
    Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Peter Zijlstra <peterz@infradead.org> - 2016-11-21 10:50 +0100
      Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-11-21 12:10 +0100
  Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Peter Zijlstra <peterz@infradead.org> - 2016-11-21 10:30 +0100
    Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Jiri Olsa <jolsa@redhat.com> - 2016-11-21 10:40 +0100
      Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Peter Zijlstra <peterz@infradead.org> - 2016-11-21 10:50 +0100
        Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Jiri Olsa <jolsa@redhat.com> - 2016-11-21 12:30 +0100
          Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Peter Zijlstra <peterz@infradead.org> - 2016-11-21 12:40 +0100
            Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Jiri Olsa <jolsa@redhat.com> - 2016-11-21 13:50 +0100
      Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Peter Zijlstra <peterz@infradead.org> - 2016-11-21 14:00 +0100
        Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Steven Rostedt <rostedt@goodmis.org> - 2016-11-21 15:30 +0100
          Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Peter Zijlstra <peterz@infradead.org> - 2016-11-21 15:40 +0100
            Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Borislav Petkov <bp@alien8.de> - 2016-11-21 16:40 +0100
              Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Peter Zijlstra <peterz@infradead.org> - 2016-11-21 16:50 +0100
                Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Borislav Petkov <bp@alien8.de> - 2016-11-21 17:10 +0100
        Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Steven Rostedt <rostedt@goodmis.org> - 2016-11-21 15:30 +0100
    Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Andi Kleen <andi@firstfloor.org> - 2016-11-21 18:10 +0100
      Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Peter Zijlstra <peterz@infradead.org> - 2016-11-21 18:20 +0100
        Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Andi Kleen <andi@firstfloor.org> - 2016-11-21 18:50 +0100
          Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Andi Kleen <andi@firstfloor.org> - 2016-11-21 19:10 +0100
            Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Steven Rostedt <rostedt@goodmis.org> - 2016-11-21 19:30 +0100
              Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Andi Kleen <andi@firstfloor.org> - 2016-11-21 19:40 +0100
                Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Steven Rostedt <rostedt@goodmis.org> - 2016-11-21 20:10 +0100
                Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Steven Rostedt <rostedt@goodmis.org> - 2016-11-21 20:20 +0100
                Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Andi Kleen <andi@firstfloor.org> - 2016-11-21 21:50 +0100
                Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-11-22 09:20 +0100
                Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Steven Rostedt <rostedt@goodmis.org> - 2016-11-22 15:40 +0100
                Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Andi Kleen <andi@firstfloor.org> - 2016-11-22 20:10 +0100
                Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Steven Rostedt <rostedt@goodmis.org> - 2016-11-24 03:10 +0100
          Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Steven Rostedt <rostedt@goodmis.org> - 2016-11-21 19:10 +0100
        Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Steven Rostedt <rostedt@goodmis.org> - 2016-11-21 19:00 +0100
          Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! Steven Rostedt <rostedt@goodmis.org> - 2016-11-21 19:30 +0100
            Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage! "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-11-21 21:20 +0100

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