Re: [PATCH] KVM guest: prevent tracing recursion with kvmclock

[Avi Kivity <avi@redhat.com>]

On 11/15/2011 03:38 PM, Steven Rostedt wrote:
> On Tue, 2011-11-15 at 15:01 +0200, Avi Kivity wrote:
> > Stop tracing when we read the clock, since tracing will also
> > want to read the clock, and recurse indefinitely.
>
> I would rephrase the above. You don't actually stop tracing, you just
> don't trace the preempt disable. I would reword that to something like:
>
> Prevent tracing of preempt_disable() in get_cpu_var() in
> kvm_clock_read(). When CONFIG_DEBUG_PREEMPT is enabled,
> preempt_disable/enable() are traced and this causes the function_graph
> tracer to go into an infinite recursion. By open coding the
> preempt_disable() around the get_cpu_var(), we can use the notrace
> version which prevents preempt_disable/enable() from being traced and
> prevents the recursion.
>
> Something like the above.

Thanks, I adopted your wording.

> > 
> > Based on a similar patch for Xen from Jeremy Fitzhardinge.
> > 
> > Signed-off-by: Avi Kivity <avi@redhat.com>
>
> This was exactly my first thought, but I was thinking it may be better
> to have a get_cpu_var_notrace() than to have to open code this stuff.
> Maybe there's not that many users that open code is not an issue. I'll
> still want to add that recursion protection with the warn on though.
>

What about function traces?  Will any noninlined calls cause the same
problem?

-- 
error compiling committee.c: too many arguments to function