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>

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index f53708be95eb30fa3f2a9096c26273228ac159ac..57b1b6811b61e656e41c5a687e58f9cc20159c79 100644 (file)
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -166,10 +166,8 @@
        CPU_KEEP(exit.data)                                             \
        MEM_KEEP(init.data)                                             \
        MEM_KEEP(exit.data)                                             \
-       . = ALIGN(32);                                                  \
-       VMLINUX_SYMBOL(__start___tracepoints) = .;                      \
+       STRUCT_ALIGN();                                                 \
        *(__tracepoints)                                                \
-       VMLINUX_SYMBOL(__stop___tracepoints) = .;                       \
        /* implement dynamic printk debug */                            \
        . = ALIGN(8);                                                   \
        VMLINUX_SYMBOL(__start___verbose) = .;                          \
@@ -218,6 +216,10 @@
                VMLINUX_SYMBOL(__start_rodata) = .;                     \
                *(.rodata) *(.rodata.*)                                 \
                *(__vermagic)           /* Kernel version magic */      \
+               . = ALIGN(8);                                           \
+               VMLINUX_SYMBOL(__start___tracepoints_ptrs) = .;         \
+               *(__tracepoints_ptrs)   /* Tracepoints: pointer array */\
+               VMLINUX_SYMBOL(__stop___tracepoints_ptrs) = .;          \
                *(__markers_strings)    /* Markers: strings */          \
                *(__tracepoints_strings)/* Tracepoints: strings */      \
        }                                                               \

diff --git a/include/linux/module.h b/include/linux/module.h
index 7695a303bb554e0840c8bf05563cb01beeb601a7..9bdf27c7615be7c4c2c25ba86312e895c8bfd866 100644 (file)
--- 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 c6814616653b1ff4d385b8b4148c4b2b14847e31..97c84a58efb8d806b1ad983a5dc7ac542768a314 100644 (file)
--- 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);

diff --git a/kernel/module.c b/kernel/module.c
index 34e00b708fad2c79b260ab3d8d4cc199cece8eca..efa290ea94bf75d843a876a0a729bbd851088429 100644 (file)
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2460,9 +2460,9 @@ static void find_module_sections(struct module *mod, struct load_info *info)
 #endif
 
 #ifdef CONFIG_TRACEPOINTS
-       mod->tracepoints = section_objs(info, "__tracepoints",
-                                       sizeof(*mod->tracepoints),
-                                       &mod->num_tracepoints);
+       mod->tracepoints_ptrs = section_objs(info, "__tracepoints_ptrs",
+                                            sizeof(*mod->tracepoints_ptrs),
+                                            &mod->num_tracepoints);
 #endif
 #ifdef HAVE_JUMP_LABEL
        mod->jump_entries = section_objs(info, "__jump_table",
@@ -3393,7 +3393,7 @@ void module_layout(struct module *mod,
                   struct modversion_info *ver,
                   struct kernel_param *kp,
                   struct kernel_symbol *ks,
-                  struct tracepoint *tp)
+                  struct tracepoint * const *tp)
 {
 }
 EXPORT_SYMBOL(module_layout);
@@ -3407,8 +3407,8 @@ void module_update_tracepoints(void)
        mutex_lock(&module_mutex);
        list_for_each_entry(mod, &modules, list)
                if (!mod->taints)
-                       tracepoint_update_probe_range(mod->tracepoints,
-                               mod->tracepoints + mod->num_tracepoints);
+                       tracepoint_update_probe_range(mod->tracepoints_ptrs,
+                               mod->tracepoints_ptrs + mod->num_tracepoints);
        mutex_unlock(&module_mutex);
 }
 
@@ -3432,8 +3432,8 @@ int module_get_iter_tracepoints(struct tracepoint_iter *iter)
                        else if (iter_mod > iter->module)
                                iter->tracepoint = NULL;
                        found = tracepoint_get_iter_range(&iter->tracepoint,
-                               iter_mod->tracepoints,
-                               iter_mod->tracepoints
+                               iter_mod->tracepoints_ptrs,
+                               iter_mod->tracepoints_ptrs
                                        + iter_mod->num_tracepoints);
                        if (found) {
                                iter->module = iter_mod;

diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index e95ee7f31d43309949893d9e63d1703e619a6ca9..68187af4889e0d303a67e8213cc64e402b437520 100644 (file)
--- 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;