tracing: Replace trace_event struct array with pointer array

Currently the trace_event structures are placed in the _ftrace_events
section, and at link time, the linker makes one large array of all
the trace_event structures. On boot up, this array is read (much like
the initcall sections) and the events are processed.

The problem is that there is no guarantee that gcc will place complex
structures nicely together in an array format. Two structures in the
same file may be placed awkwardly, because gcc has no clue that they
are suppose to be in an array.

A hack was used previous to force the alignment to 4, to pack the
structures together. But this caused alignment issues with other
architectures (sparc).

Instead of packing the structures into an array, the structures' addresses
are now put into the _ftrace_event section. As pointers are always the
natural alignment, gcc should always pack them tightly together
(otherwise initcall, extable, etc would also fail).

By having the pointers to the structures in the section, we can still
iterate the trace_events without causing unnecessary alignment problems
with other architectures, or depending on the current behaviour of
gcc that will likely change in the future just to tick us kernel developers
off a little more.

The _ftrace_event section is also moved into the .init.data section
as it is now only needed at boot up.

Suggested-by: David Miller <davem@davemloft.net>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 13 insertions, 11 deletions

diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
index e16610c208c9..3e68366d485a 100644
--- a/include/trace/ftrace.h
+++ b/include/trace/ftrace.h
@@ -446,14 +446,16 @@ static inline notrace int ftrace_get_offsets_##call(			\
  *      .reg                    = ftrace_event_reg,
  * };
  *
- * static struct ftrace_event_call __used
- * __attribute__((__aligned__(4)))
- * __attribute__((section("_ftrace_events"))) event_<call> = {
+ * static struct ftrace_event_call event_<call> = {
  *      .name                   = "<call>",
  *      .class                  = event_class_<template>,
  *      .event                  = &ftrace_event_type_<call>,
  *      .print_fmt              = print_fmt_<call>,
  * };
+ * // its only safe to use pointers when doing linker tricks to
+ * // create an array.
+ * static struct ftrace_event_call __used
+ * __attribute__((section("_ftrace_events"))) *__event_<call> = &event_<call>;
  *
  */
 
@@ -579,28 +581,28 @@ static struct ftrace_event_class __used event_class_##call = {		\
 #undef DEFINE_EVENT
 #define DEFINE_EVENT(template, call, proto, args)                       \
                                                                         \
-static struct ftrace_event_call __used                                  \
-__attribute__((__aligned__(4)))                                         \
-__attribute__((section("_ftrace_events"))) event_##call = {             \
+static struct ftrace_event_call __used event_##call = {                 \
         .name                   = #call,                                \
         .class                  = &event_class_##template,              \
         .event.funcs            = &ftrace_event_type_funcs_##template,  \
         .print_fmt              = print_fmt_##template,                 \
-};
+};                                                                      \
+static struct ftrace_event_call __used                                  \
+__attribute__((section("_ftrace_events"))) *__event_##call = &event_##call
 
 #undef DEFINE_EVENT_PRINT
 #define DEFINE_EVENT_PRINT(template, call, proto, args, print)          \
                                                                         \
 static const char print_fmt_##call[] = print;                           \
                                                                         \
-static struct ftrace_event_call __used                                  \
-__attribute__((__aligned__(4)))                                         \
-__attribute__((section("_ftrace_events"))) event_##call = {             \
+static struct ftrace_event_call __used event_##call = {                 \
         .name                   = #call,                                \
         .class                  = &event_class_##template,              \
         .event.funcs            = &ftrace_event_type_funcs_##call,      \
         .print_fmt              = print_fmt_##call,                     \
-}
+};                                                                      \
+static struct ftrace_event_call __used                                  \
+__attribute__((section("_ftrace_events"))) *__event_##call = &event_##call
 
 #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)