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    SubjectRe: [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@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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