Re: [PATCH 3/7] add syscall tracepoints

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Jason Baron (jbaron@redhat.com) wrote:
> On Fri, Jun 12, 2009 at 05:57:26PM -0400, Mathieu Desnoyers wrote:
> > > Introduce a new 'DECLARE_TRACE_REG()' macro, so that tracepoints can associate
> > > an external register/unregister function.
> > > 
> > > 
> > > Signed-off-by: Jason Baron <jbaron@redhat.com>
> > > 
> > > ---
> > >  include/linux/tracepoint.h |   27 +++++++++++++++++++++++----
> > >  1 files changed, 23 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > > index 14df7e6..9a3660b 100644
> > > --- a/include/linux/tracepoint.h
> > > +++ b/include/linux/tracepoint.h
> > > @@ -61,7 +61,7 @@ struct tracepoint {
> > >   * not add unwanted padding between the beginning of the section and the
> > >   * structure. Force alignment to the same alignment as the section start.
> > >   */
> > > -#define DECLARE_TRACE(name, proto, args)				\
> > > +#define DECLARE_TRACE_REG(name, proto, args, reg, unreg)		\
> > >  	extern struct tracepoint __tracepoint_##name;			\
> > >  	static inline void trace_##name(proto)				\
> > >  	{								\
> > > @@ -71,13 +71,29 @@ struct tracepoint {
> > >  	}								\
> > >  	static inline int register_trace_##name(void (*probe)(proto))	\
> > >  	{								\
> > > -		return tracepoint_probe_register(#name, (void *)probe);	\
> > > +		int ret;						\
> > > +		void (*func)(void) = (void (*)(void))reg;		\
> > > +									\
> > > +		ret = tracepoint_probe_register(#name, (void *)probe);	\
> > > +		if (func && !ret)					\
> > > +			func();						\
> > 
> > I don't see why you need to add this weird interface when all you really
> > need to do is to call the function to set the TIF flags explicitly in
> > reg_event_syscall_enter when registering a tracepoint.
> > 
> > Mathieu
> > 
> 
> I could enable the TIF flag in reg_event_syscall_enter, however that
> would not manage the TIF flag for other users of these traceoints. When
> users 'register/unregister' with a tracepoint, they expect the tracepoint
> to be enabled/disabled. If we move this functionality to the user, we are
> changing how that interface works. Therefore, I associated the
> enabling/disabling of the tracepoint, with the tracepoint definition.
> 

I agree it should be hidden from userspace APIs, but I don't think we
should hide it or from the "in kernel" API users, really. People
interfacing with this kind of API from the kernel-side expect to have a
great level of control on how they use it, and we can expect people to
know what they are doing.

Mathieu


> thanks,
> 
> -Jason

-- 
Mathieu Desnoyers
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