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>

1 files changed, 16 insertions, 15 deletions

diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index e95ee7f31d43..68187af4889e 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -27,8 +27,8 @@
 #include <linux/sched.h>
 #include <linux/jump_label.h>
 
-extern struct tracepoint __start___tracepoints[];
-extern struct tracepoint __stop___tracepoints[];
+extern struct tracepoint * const __start___tracepoints_ptrs[];
+extern struct tracepoint * const __stop___tracepoints_ptrs[];
 
 /* Set to 1 to enable tracepoint debug output */
 static const int tracepoint_debug;
@@ -298,10 +298,10 @@ static void disable_tracepoint(struct tracepoint *elem)
  *
  * Updates the probe callback corresponding to a range of tracepoints.
  */
-void
-tracepoint_update_probe_range(struct tracepoint *begin, struct tracepoint *end)
+void tracepoint_update_probe_range(struct tracepoint * const *begin,
+                                   struct tracepoint * const *end)
 {
-        struct tracepoint *iter;
+        struct tracepoint * const *iter;
         struct tracepoint_entry *mark_entry;
 
         if (!begin)
@@ -309,12 +309,12 @@ tracepoint_update_probe_range(struct tracepoint *begin, struct tracepoint *end)
 
         mutex_lock(&tracepoints_mutex);
         for (iter = begin; iter < end; iter++) {
-                mark_entry = get_tracepoint(iter->name);
+                mark_entry = get_tracepoint((*iter)->name);
                 if (mark_entry) {
-                        set_tracepoint(&mark_entry, iter,
+                        set_tracepoint(&mark_entry, *iter,
                                         !!mark_entry->refcount);
                 } else {
-                        disable_tracepoint(iter);
+                        disable_tracepoint(*iter);
                 }
         }
         mutex_unlock(&tracepoints_mutex);
@@ -326,8 +326,8 @@ tracepoint_update_probe_range(struct tracepoint *begin, struct tracepoint *end)
 static void tracepoint_update_probes(void)
 {
         /* Core kernel tracepoints */
-        tracepoint_update_probe_range(__start___tracepoints,
-                __stop___tracepoints);
+        tracepoint_update_probe_range(__start___tracepoints_ptrs,
+                __stop___tracepoints_ptrs);
         /* tracepoints in modules. */
         module_update_tracepoints();
 }
@@ -514,8 +514,8 @@ EXPORT_SYMBOL_GPL(tracepoint_probe_update_all);
  * Will return the first tracepoint in the range if the input tracepoint is
  * NULL.
  */
-int tracepoint_get_iter_range(struct tracepoint **tracepoint,
-        struct tracepoint *begin, struct tracepoint *end)
+int tracepoint_get_iter_range(struct tracepoint * const **tracepoint,
+        struct tracepoint * const *begin, struct tracepoint * const *end)
 {
         if (!*tracepoint && begin != end) {
                 *tracepoint = begin;
@@ -534,7 +534,8 @@ static void tracepoint_get_iter(struct tracepoint_iter *iter)
         /* Core kernel tracepoints */
         if (!iter->module) {
                 found = tracepoint_get_iter_range(&iter->tracepoint,
-                                __start___tracepoints, __stop___tracepoints);
+                                __start___tracepoints_ptrs,
+                                __stop___tracepoints_ptrs);
                 if (found)
                         goto end;
         }
@@ -585,8 +586,8 @@ int tracepoint_module_notify(struct notifier_block *self,
         switch (val) {
         case MODULE_STATE_COMING:
         case MODULE_STATE_GOING:
-                tracepoint_update_probe_range(mod->tracepoints,
-                        mod->tracepoints + mod->num_tracepoints);
+                tracepoint_update_probe_range(mod->tracepoints_ptrs,
+                        mod->tracepoints_ptrs + mod->num_tracepoints);
                 break;
         }
         return 0;