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SubjectRe: [GIT PULL] updates for oprofile
(cc'ing oprofile-list)

On 27.04.10 18:25:44, Phil Carmody wrote:
> Ingo, et al.,
>
> Regarding today's pulled request, containing:
>
> commit bc078e4eab65f11bbaeed380593ab8151b30d703
> Author: Martin Schwidefsky <schwidef...@de.ibm.com>
> Date: Tue Mar 2 16:01:10 2010 +0100
>
> oprofile: convert oprofile from timer_hook to hrtimer
>
>
> Information is a touch scant, as I'm doing the investigation as I
> write, but I believe that that patch can cause ooops regressions
> via a null-pointer dereference in oprofile_add_sample().
>
> That function declares:
> """
> /**
> * Add a sample. This may be called from any context.
> */
> void oprofile_add_sample(struct pt_regs * const regs, unsigned long event);
> """
>
> And begins:
> """
> void oprofile_add_sample(struct pt_regs * const regs, unsigned long event)
> {
> int is_kernel = !user_mode(regs);
> """
>
> Where on at least two major architectures (Arm, x86), user_mode()
> unconditionally dereferences its parameter.
>
> Now oprofile_add_sample() is called from this context:
> """
> static enum hrtimer_restart oprofile_hrtimer_notify(struct hrtimer *hrtimer)
> {
> oprofile_add_sample(get_irq_regs(), 0);
> """
>
> And get_irq_regs() is NULL when not in an IRQ context.

Perf is simply dropping the sample in such cases, see:

kernel/perf_event.c:perf_swevent_hrtimer()

So at quick fix would be to check for a null pointer also. But,
according to this:

http://www.mail-archive.com/linux-omap@vger.kernel.org/msg14074.html

samples will be incorrect then since only interrupt context is
profiled. It seems there is no solution available right now.

-Robert

>
> Bang.
>
> An example of this kind of thing kicking in has already been encountered
> last year:
> http://www.mail-archive.com/linux-omap@vger.kernel.org/msg14069.html
> (That thread got a little side-tracked onto OMAP specifics, but the
> original report is topical.)
>
> Now would be a very good time for the "many eyes" principle to kick in.
>
> I'm now looking into workarounds, but nothing that I'd necessarily
> want to submit as a real fix.
>
> Phil
> cc:'d replies appreciated
>

--
Advanced Micro Devices, Inc.
Operating System Research Center
email: robert.richter@amd.com



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