Re: [PATCH] tracepoint: Do not fail unregistering a probe due to memory allocation

Steven Rostedt
2020-11-17T19:22:10Z

On Tue, 17 Nov 2020 14:15:10 -0500 (EST)
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> diff --git a/include/linux/tracepoint-defs.h b/include/linux/tracepoint-defs.h
> index e7c2276be33e..e0351bb0b140 100644
> --- a/include/linux/tracepoint-defs.h
> +++ b/include/linux/tracepoint-defs.h
> @@ -38,6 +38,7 @@ struct tracepoint {
> int (*regfunc)(void);
> void (*unregfunc)(void);
> struct tracepoint_func __rcu *funcs;
> + void *stub_func;
> };
>
> #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index 0f21617f1a66..b0b805de3779 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -287,6 +287,9 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> #define DEFINE_TRACE_FN(_name, _reg, _unreg, proto, args) \
> static const char __tpstrtab_##_name[] \
> __section("__tracepoints_strings") = #_name; \
> + static void __cold __tracepoint_stub_func_##_name(void *__data, proto) \
> + { \
> + } \

The thing is, tracepoints are already bloated. I do not want to add
something like this that will unnecessarily add more text.

Since all tracepoints callbacks have at least one parameter (__data), we
could declare tp_stub_func as:

static void tp_stub_func(void *data, ...)
{
return;
}

And now C knows that tp_stub_func() can be called with one or more
parameters, and had better be able to deal with it!

-- Steve

> extern struct static_call_key STATIC_CALL_KEY(tp_func_##_name); \
> int __traceiter_##_name(void *__data, proto); \
> struct tracepoint __tracepoint_##_name __used \
> @@ -298,7 +301,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> .iterator = &__traceiter_##_name, \
> .regfunc = _reg, \
> .unregfunc = _unreg, \
> - .funcs = NULL }; \
> + .funcs = NULL, \
> + .stub_func = __tracepoint_stub_func_##_name, }; \
> __TRACEPOINT_ENTRY(_name); \
> int __traceiter_##_name(void *__data, proto) \
> { \
>
> Thanks,
>
> Mathieu
>