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SubjectRe: [PATCH 2/2] Tracepoints: Fix section alignment using pointer array
* Steven Rostedt (rostedt@goodmis.org) wrote:
> From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>
> 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 7e066fb870fcd1025ec3ba7bbde5d541094f4ce1 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.
>
> commit 15e3540ce2159705f18fad6147ffedf04445ad64 tried 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.

Small nit: this reference to commit 15e3540ce2159705f18fad6147ffedf04445ad64
should be changed to a non-commit-id-related explanation, because the commit has
been rolled back from -tip.

The rest looks good.

Thanks,

Mathieu

>
> 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>
> LKML-Reference: <20110126222622.GA10794@Krystal>
> CC: David Miller <davem@davemloft.net>
> 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>
> ---
> include/asm-generic/vmlinux.lds.h | 8 +++++---
> include/linux/module.h | 2 +-
> include/linux/tracepoint.h | 35 ++++++++++++++++++++---------------
> kernel/module.c | 16 ++++++++--------
> kernel/tracepoint.c | 31 ++++++++++++++++---------------
> 5 files changed, 50 insertions(+), 42 deletions(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index f53708b..57b1b68 100644
> --- 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 7695a30..9bdf27c 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 c681461..97c84a5 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);
> diff --git a/kernel/module.c b/kernel/module.c
> index 34e00b7..efa290e 100644
> --- 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 e95ee7f..68187af 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;
> --
> 1.7.2.3
>
>

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com


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