tracepoints: Fix section alignment using pointer array

Make the tracepoints more robust, making them solid enough to handle compiler
changes by not relying on anything based on compiler-specific behavior with
respect to structure alignment. Implement an approach proposed by David Miller:
use an array of const pointers to refer to the individual structures, and export
this pointer array through the linker script rather than the structures per se.
It will consume 32 extra bytes per tracepoint (24 for structure padding and 8
for the pointers), but are less likely to break due to compiler changes.

History:

commit 7e066fb8 tracepoints: add DECLARE_TRACE() and DEFINE_TRACE()
added the aligned(32) type and variable attribute to the tracepoint structures
to deal with gcc happily aligning statically defined structures on 32-byte
multiples.

One attempt was to use a 8-byte alignment for tracepoint structures by applying
both the variable and type attribute to tracepoint structures definitions and
declarations. It worked fine with gcc 4.5.1, but broke with gcc 4.4.4 and 4.4.5.

The reason is that the "aligned" attribute only specify the _minimum_ alignment
for a structure, leaving both the compiler and the linker free to align on
larger multiples. Because tracepoint.c expects the structures to be placed as an
array within each section, up-alignment cause NULL-pointer exceptions due to the
extra unexpected padding.

(this patch applies on top of -tip)

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: David S. Miller <davem@davemloft.net>
LKML-Reference: <20110126222622.GA10794@Krystal>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Ingo Molnar <mingo@elte.hu>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

2 files changed, 21 insertions, 16 deletions

diff --git a/include/linux/module.h b/include/linux/module.h
index 7695a303bb55..9bdf27c7615b 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -377,7 +377,7 @@ struct module
            keeping pointers to this stuff */
         char *args;
 #ifdef CONFIG_TRACEPOINTS
-        struct tracepoint *tracepoints;
+        struct tracepoint * const *tracepoints_ptrs;
         unsigned int num_tracepoints;
 #endif
 #ifdef HAVE_JUMP_LABEL
diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index c6814616653b..97c84a58efb8 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -33,12 +33,7 @@ struct tracepoint {
         void (*regfunc)(void);
         void (*unregfunc)(void);
         struct tracepoint_func __rcu *funcs;
-} __attribute__((aligned(32)));         /*
-                                         * Aligned on 32 bytes because it is
-                                         * globally visible and gcc happily
-                                         * align these on the structure size.
-                                         * Keep in sync with vmlinux.lds.h.
-                                         */
+};
 
 /*
  * Connect a probe to a tracepoint.
@@ -61,15 +56,15 @@ extern void tracepoint_probe_update_all(void);
 
 struct tracepoint_iter {
         struct module *module;
-        struct tracepoint *tracepoint;
+        struct tracepoint * const *tracepoint;
 };
 
 extern void tracepoint_iter_start(struct tracepoint_iter *iter);
 extern void tracepoint_iter_next(struct tracepoint_iter *iter);
 extern void tracepoint_iter_stop(struct tracepoint_iter *iter);
 extern void tracepoint_iter_reset(struct tracepoint_iter *iter);
-extern int tracepoint_get_iter_range(struct tracepoint **tracepoint,
-        struct tracepoint *begin, struct tracepoint *end);
+extern int tracepoint_get_iter_range(struct tracepoint * const **tracepoint,
+        struct tracepoint * const *begin, struct tracepoint * const *end);
 
 /*
  * tracepoint_synchronize_unregister must be called between the last tracepoint
@@ -84,11 +79,13 @@ static inline void tracepoint_synchronize_unregister(void)
 #define PARAMS(args...) args
 
 #ifdef CONFIG_TRACEPOINTS
-extern void tracepoint_update_probe_range(struct tracepoint *begin,
-        struct tracepoint *end);
+extern
+void tracepoint_update_probe_range(struct tracepoint * const *begin,
+        struct tracepoint * const *end);
 #else
-static inline void tracepoint_update_probe_range(struct tracepoint *begin,
-        struct tracepoint *end)
+static inline
+void tracepoint_update_probe_range(struct tracepoint * const *begin,
+        struct tracepoint * const *end)
 { }
 #endif /* CONFIG_TRACEPOINTS */
 
@@ -174,12 +171,20 @@ do_trace:								\
         {                                                               \
         }
 
+/*
+ * We have no guarantee that gcc and the linker won't up-align the tracepoint
+ * structures, so we create an array of pointers that will be used for iteration
+ * on the tracepoints.
+ */
 #define DEFINE_TRACE_FN(name, reg, unreg)                               \
         static const char __tpstrtab_##name[]                           \
         __attribute__((section("__tracepoints_strings"))) = #name;      \
         struct tracepoint __tracepoint_##name                           \
-        __attribute__((section("__tracepoints"), aligned(32))) =        \
-                { __tpstrtab_##name, 0, reg, unreg, NULL }
+        __attribute__((section("__tracepoints"))) =                     \
+                { __tpstrtab_##name, 0, reg, unreg, NULL };             \
+        static struct tracepoint * const __tracepoint_ptr_##name __used \
+        __attribute__((section("__tracepoints_ptrs"))) =                \
+                &__tracepoint_##name;
 
 #define DEFINE_TRACE(name)                                              \
         DEFINE_TRACE_FN(name, NULL, NULL);