Re: [PATCH 05/05] Linux Kernel Markers, non optimised architectures

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

* Nick Piggin (nickpiggin@yahoo.com.au) wrote:
> Mathieu Desnoyers wrote:
> 
> >+#define MARK(name, format, args...) \
> >+	do { \
> >+		static marker_probe_func *__mark_call_##name = \
> >+					__mark_empty_function; \
> >+		volatile static char __marker_enable_##name = 0; \
> >+		static const struct __mark_marker_c __mark_c_##name \
> >+			__attribute__((section(".markers.c"))) = \
> >+			{ #name, &__mark_call_##name, format } ; \
> >+		static const struct __mark_marker __mark_##name \
> >+			__attribute__((section(".markers"))) = \
> >+			{ &__mark_c_##name, &__marker_enable_##name } ; \
> >+		asm volatile ( "" : : "i" (&__mark_##name)); \
> >+		__mark_check_format(format, ## args); \
> >+		if (unlikely(__marker_enable_##name)) { \
> >+			preempt_disable(); \
> >+			(*__mark_call_##name)(format, ## args); \
> >+			preempt_enable_no_resched(); \
> 
> Why not just preempt_enable() here?
> 

Because the preempt_enable() macro contains preempt_check_resched(), which
may call preempt_schedule() which leads us to a call to schedule(). Therefore,
all those very interesting scheduler functions would cause an infinite
recursive scheduler call if we marked schedule() and used preempt_enable() in
the marker.

The primary goal for the markers (and the probes that attaches to them) is to
have the fewest side-effects possible : any kernel method called from an
instrumentation site adds this precise kernel method to the "cannot be
instrumented" list, which I want to keep as small possible.

Mathieu

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