Re: [PATCH] oprofile: fix CPU unplug panic in ppro_stop()

[Andi Kleen]

Robert Richter wrote:
> On 02.12.08 09:17:29, Ingo Molnar wrote:
> > * Eric Dumazet <dada1@xxxxxxxxxxxxx> wrote:
> >
> > > If oprofile statically compiled in kernel, a cpu unplug triggers
> > > a panic in ppro_stop(), because a NULL pointer is dereferenced.
> > >
> > > Signed-off-by: Eric Dumazet <dada1@xxxxxxxxxxxxx>
> > > ---
> > > arch/x86/oprofile/op_model_ppro.c |    4 ++++
> > > 1 files changed, 4 insertions(+)
> > > diff --git a/arch/x86/oprofile/op_model_ppro.c b/arch/x86/oprofile/op_model_ppro.c
> > > index 716d26f..e9f80c7 100644
> > > --- a/arch/x86/oprofile/op_model_ppro.c
> > > +++ b/arch/x86/oprofile/op_model_ppro.c
> > > @@ -156,6 +156,8 @@ static void ppro_start(struct op_msrs const * const msrs)
> > >  	unsigned int low, high;
> > >  	int i;
> > >  
> > > +	if (!reset_value)
> > > +		return;
> > >
> > > 	for (i = 0; i < num_counters; ++i) {
> > >  		if (reset_value[i]) {
> > >  			CTRL_READ(low, high, msrs, i);
>
> The patch fixes the null pointer access and this ok. But the root
> cause seems to be in the cpu hotplug and initialization
> code. xxx_start() should not be called before xxx_setup_ctrs() or
> after xxx_shutdown(). 

Yes, it would be better to fix that. At least it would make
the code cleaner than the add checks for this backdoor everywhere.

> Also, running only xxx_start() and xxx_stop() in
> the cpu notifier functions is not sufficient. There is at least some
> on_each_cpu code in nmi_setup() that should be called also in the cpu
> notifier functions. I have to review that code.

AFAIK cpu hotplug has more problems in oprofile anyways. That is why
I didn't test that case.

> [...]
>
> > It was absolutely unnecessary to add kmalloc to this rarely executed 
> > codepath - and the way it was added was absolutely horrible as well, it 
> > was tacked on in the middle of an existing codepath, instead of factoring 
> > it out nicely. Perfmon will eventually replace PMC management anyway, so 
> > there was no "this way it's cleaner" argument either. So this code should 
> > have been changed minimally, instead of slapping in a full kmalloc for a 
> > simple array extension from 2 to 4 entries ...
>
> Ingo, you are right that using kmalloc is unnecessary for
> reset_value. So, Andi, maybe you could make this code easier?

The reason I added the kmalloc is that there's also a varying number
of separate fixed function counters (although that's not currently
submitted).

Also I would prefer to not have a hard coded number for future
CPUs. Contrary to other people's opinion architectural perfmon is
not for Nehalem only.

-Andi
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