[RFC PATCH 3/3] Tracepoint: register/unregister struct tracepoint

[Mathieu Desnoyers]

Register/unregister tracepoint probes with struct tracepoint pointer
rather than tracepoint name.

This change, which vastly simplifies tracepoint.c, has been proposed by
Steven Rostedt.

>From this point on, the tracers need to pass a struct tracepoint pointer
to probe register/unregister. A probe can now only be connected to a
tracepoint that exists. Moreover, tracers are responsible for
unregistering the probe before the module containing its associated
tracepoint is unloaded.

This patch is currently only compile-tested for kernel/tracepoint.o. The
callers still need to be adapted.

Not-Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: Frank Ch. Eigler <fche@xxxxxxxxxx>
CC: Johannes Berg <johannes.berg@xxxxxxxxx>
---
 include/linux/tracepoint.h |    7 +-
 kernel/tracepoint.c        |  440 +++++++++++---------------------------------
 2 files changed, 116 insertions(+), 331 deletions(-)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 812b255..f7bbd63 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -39,14 +39,15 @@ struct tracepoint {
  * Connect a probe to a tracepoint.
  * Internal API, should not be used directly.
  */
-extern int tracepoint_probe_register(const char *name, void *probe, void *data);
+extern int tracepoint_probe_register(struct tracepoint *tp, void *probe,
+		void *data);
 
 /*
  * Disconnect a probe from a tracepoint.
  * Internal API, should not be used directly.
  */
-extern int
-tracepoint_probe_unregister(const char *name, void *probe, void *data);
+extern int tracepoint_probe_unregister(struct tracepoint *tp, void *probe,
+		void *data);
 
 #ifdef CONFIG_MODULES
 struct tp_module {
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index cc6c8b6..97f0517 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -27,44 +27,19 @@
 #include <linux/sched.h>
 #include <linux/static_key.h>
 
-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;
 
 /*
- * Tracepoints mutex protects the builtin and module tracepoints and the hash
- * table, as well as the local module list.
+ * Tracepoints mutex protects the builtin and module tracepoints.
  */
 static DEFINE_MUTEX(tracepoints_mutex);
 
-#ifdef CONFIG_MODULES
-/* Local list of struct module */
-static LIST_HEAD(tracepoint_module_list);
-#endif /* CONFIG_MODULES */
-
-/*
- * Tracepoint hash table, containing the active tracepoints.
- * Protected by tracepoints_mutex.
- */
-#define TRACEPOINT_HASH_BITS 6
-#define TRACEPOINT_TABLE_SIZE (1 << TRACEPOINT_HASH_BITS)
-static struct hlist_head tracepoint_table[TRACEPOINT_TABLE_SIZE];
-
 /*
  * Note about RCU :
  * It is used to delay the free of multiple probes array until a quiescent
  * state is reached.
- * Tracepoint entries modifications are protected by the tracepoints_mutex.
  */
-struct tracepoint_entry {
-	struct hlist_node hlist;
-	struct tracepoint_func *funcs;
-	int refcount;	/* Number of times armed. 0 if disarmed. */
-	char name[0];
-};
-
 struct tp_probes {
 	union {
 		struct rcu_head rcu;
@@ -94,34 +69,35 @@ static inline void release_probes(struct tracepoint_func *old)
 	}
 }
 
-static void debug_print_probes(struct tracepoint_entry *entry)
+static
+void debug_print_probes(struct tracepoint_func *funcs)
 {
 	int i;
 
-	if (!tracepoint_debug || !entry->funcs)
+	if (!tracepoint_debug || !funcs)
 		return;
 
-	for (i = 0; entry->funcs[i].func; i++)
-		printk(KERN_DEBUG "Probe %d : %p\n", i, entry->funcs[i].func);
+	for (i = 0; funcs[i].func; i++)
+		printk(KERN_DEBUG "Probe %d : %p\n", i, funcs[i].func);
 }
 
-static struct tracepoint_func *
-tracepoint_entry_add_probe(struct tracepoint_entry *entry,
-			   void *probe, void *data)
+static
+struct tracepoint_func *func_add(struct tracepoint_func **funcs,
+		struct tracepoint_func *tp_func)
 {
 	int nr_probes = 0;
 	struct tracepoint_func *old, *new;
 
-	if (WARN_ON(!probe))
+	if (WARN_ON(!tp_func->func))
 		return ERR_PTR(-EINVAL);
 
-	debug_print_probes(entry);
-	old = entry->funcs;
+	debug_print_probes(*funcs);
+	old = *funcs;
 	if (old) {
 		/* (N -> N+1), (N != 0, 1) probes */
 		for (nr_probes = 0; old[nr_probes].func; nr_probes++)
-			if (old[nr_probes].func == probe &&
-			    old[nr_probes].data == data)
+			if (old[nr_probes].func == tp_func->func &&
+			    old[nr_probes].data == tp_func->data)
 				return ERR_PTR(-EEXIST);
 	}
 	/* + 2 : one for new probe, one for NULL func */
@@ -130,33 +106,31 @@ tracepoint_entry_add_probe(struct tracepoint_entry *entry,
 		return ERR_PTR(-ENOMEM);
 	if (old)
 		memcpy(new, old, nr_probes * sizeof(struct tracepoint_func));
-	new[nr_probes].func = probe;
-	new[nr_probes].data = data;
+	new[nr_probes] = *tp_func;
 	new[nr_probes + 1].func = NULL;
-	entry->refcount = nr_probes + 1;
-	entry->funcs = new;
-	debug_print_probes(entry);
+	*funcs = new;
+	debug_print_probes(*funcs);
 	return old;
 }
 
-static void *
-tracepoint_entry_remove_probe(struct tracepoint_entry *entry,
-			      void *probe, void *data)
+static
+void *func_remove(struct tracepoint_func **funcs,
+		struct tracepoint_func *tp_func)
 {
 	int nr_probes = 0, nr_del = 0, i;
 	struct tracepoint_func *old, *new;
 
-	old = entry->funcs;
+	old = *funcs;
 
 	if (!old)
 		return ERR_PTR(-ENOENT);
 
-	debug_print_probes(entry);
+	debug_print_probes(*funcs);
 	/* (N -> M), (N > 1, M >= 0) probes */
-	if (probe) {
+	if (tp_func->func) {
 		for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
-			if (old[nr_probes].func == probe &&
-			     old[nr_probes].data == data)
+			if (old[nr_probes].func == tp_func->func &&
+			     old[nr_probes].data == tp_func->data)
 				nr_del++;
 		}
 	}
@@ -167,9 +141,8 @@ tracepoint_entry_remove_probe(struct tracepoint_entry *entry,
 	 */
 	if (nr_probes - nr_del == 0) {
 		/* N -> 0, (N > 1) */
-		entry->funcs = NULL;
-		entry->refcount = 0;
-		debug_print_probes(entry);
+		*funcs = NULL;
+		debug_print_probes(*funcs);
 		return old;
 	} else {
 		int j = 0;
@@ -179,90 +152,35 @@ tracepoint_entry_remove_probe(struct tracepoint_entry *entry,
 		if (new == NULL)
 			return ERR_PTR(-ENOMEM);
 		for (i = 0; old[i].func; i++)
-			if (old[i].func != probe || old[i].data != data)
+			if (old[i].func != tp_func->func
+					|| old[i].data != tp_func->data)
 				new[j++] = old[i];
 		new[nr_probes - nr_del].func = NULL;
-		entry->refcount = nr_probes - nr_del;
-		entry->funcs = new;
+		*funcs = new;
 	}
-	debug_print_probes(entry);
+	debug_print_probes(*funcs);
 	return old;
 }
 
 /*
- * Get tracepoint if the tracepoint is present in the tracepoint hash table.
- * Must be called with tracepoints_mutex held.
- * Returns NULL if not present.
+ * Add the probe function to a tracepoint.
  */
-static struct tracepoint_entry *get_tracepoint(const char *name)
+static
+int tracepoint_add_func(struct tracepoint *tp,
+		struct tracepoint_func *func)
 {
-	struct hlist_head *head;
-	struct tracepoint_entry *e;
-	u32 hash = jhash(name, strlen(name), 0);
-
-	head = &tracepoint_table[hash & (TRACEPOINT_TABLE_SIZE - 1)];
-	hlist_for_each_entry(e, head, hlist) {
-		if (!strcmp(name, e->name))
-			return e;
-	}
-	return NULL;
-}
+	struct tracepoint_func *old, *tp_funcs;
 
-/*
- * Add the tracepoint to the tracepoint hash table. Must be called with
- * tracepoints_mutex held.
- */
-static struct tracepoint_entry *add_tracepoint(const char *name)
-{
-	struct hlist_head *head;
-	struct tracepoint_entry *e;
-	size_t name_len = strlen(name) + 1;
-	u32 hash = jhash(name, name_len-1, 0);
-
-	head = &tracepoint_table[hash & (TRACEPOINT_TABLE_SIZE - 1)];
-	hlist_for_each_entry(e, head, hlist) {
-		if (!strcmp(name, e->name)) {
-			printk(KERN_NOTICE
-				"tracepoint %s busy\n", name);
-			return ERR_PTR(-EEXIST);	/* Already there */
-		}
-	}
-	/*
-	 * Using kmalloc here to allocate a variable length element. Could
-	 * cause some memory fragmentation if overused.
-	 */
-	e = kmalloc(sizeof(struct tracepoint_entry) + name_len, GFP_KERNEL);
-	if (!e)
-		return ERR_PTR(-ENOMEM);
-	memcpy(&e->name[0], name, name_len);
-	e->funcs = NULL;
-	e->refcount = 0;
-	hlist_add_head(&e->hlist, head);
-	return e;
-}
+	if (tp->regfunc && !static_key_enabled(&tp->key))
+		tp->regfunc();
 
-/*
- * Remove the tracepoint from the tracepoint hash table. Must be called with
- * mutex_lock held.
- */
-static inline void remove_tracepoint(struct tracepoint_entry *e)
-{
-	hlist_del(&e->hlist);
-	kfree(e);
-}
-
-/*
- * Sets the probe callback corresponding to one tracepoint.
- */
-static void set_tracepoint(struct tracepoint_entry **entry,
-	struct tracepoint *elem, int active)
-{
-	WARN_ON(strcmp((*entry)->name, elem->name) != 0);
-
-	if (elem->regfunc && !static_key_enabled(&elem->key) && active)
-		elem->regfunc();
-	else if (elem->unregfunc && static_key_enabled(&elem->key) && !active)
-		elem->unregfunc();
+	tp_funcs = tp->funcs;
+	old = func_add(&tp_funcs, func);
+	if (IS_ERR(old)) {
+		WARN_ON_ONCE(1);
+		return PTR_ERR(old);
+	}
+	release_probes(old);
 
 	/*
 	 * rcu_assign_pointer has a smp_wmb() which makes sure that the new
@@ -271,272 +189,138 @@ static void set_tracepoint(struct tracepoint_entry **entry,
 	 * include/linux/tracepoints.h. A matching smp_read_barrier_depends()
 	 * is used.
 	 */
-	rcu_assign_pointer(elem->funcs, (*entry)->funcs);
-	if (active && !static_key_enabled(&elem->key))
-		static_key_slow_inc(&elem->key);
-	else if (!active && static_key_enabled(&elem->key))
-		static_key_slow_dec(&elem->key);
+	rcu_assign_pointer(tp->funcs, tp_funcs);
+	if (!static_key_enabled(&tp->key))
+		static_key_slow_inc(&tp->key);
+	return 0;
 }
 
 /*
- * Disable a tracepoint and its probe callback.
+ * Remove a probe function from a tracepoint.
  * Note: only waiting an RCU period after setting elem->call to the empty
  * function insures that the original callback is not used anymore. This insured
  * by preempt_disable around the call site.
  */
-static void disable_tracepoint(struct tracepoint *elem)
-{
-	if (elem->unregfunc && static_key_enabled(&elem->key))
-		elem->unregfunc();
-
-	if (static_key_enabled(&elem->key))
-		static_key_slow_dec(&elem->key);
-	rcu_assign_pointer(elem->funcs, NULL);
-}
-
-/**
- * tracepoint_update_probe_range - Update a probe range
- * @begin: beginning of the range
- * @end: end of the range
- *
- * Updates the probe callback corresponding to a range of tracepoints.
- * Called with tracepoints_mutex held.
- */
-static void tracepoint_update_probe_range(struct tracepoint * const *begin,
-					  struct tracepoint * const *end)
+static
+int tracepoint_remove_func(struct tracepoint *tp,
+		struct tracepoint_func *func)
 {
-	struct tracepoint * const *iter;
-	struct tracepoint_entry *mark_entry;
-
-	if (!begin)
-		return;
+	struct tracepoint_func *old, *tp_funcs;
 
-	for (iter = begin; iter < end; iter++) {
-		mark_entry = get_tracepoint((*iter)->name);
-		if (mark_entry) {
-			set_tracepoint(&mark_entry, *iter,
-					!!mark_entry->refcount);
-		} else {
-			disable_tracepoint(*iter);
-		}
+	tp_funcs = tp->funcs;
+	old = func_remove(&tp_funcs, func);
+	if (IS_ERR(old)) {
+		WARN_ON_ONCE(1);
+		return PTR_ERR(old);
 	}
-}
-
-#ifdef CONFIG_MODULES
-void module_update_tracepoints(void)
-{
-	struct tp_module *tp_mod;
-
-	list_for_each_entry(tp_mod, &tracepoint_module_list, list)
-		tracepoint_update_probe_range(tp_mod->tracepoints_ptrs,
-			tp_mod->tracepoints_ptrs + tp_mod->num_tracepoints);
-}
-#else /* CONFIG_MODULES */
-void module_update_tracepoints(void)
-{
-}
-#endif /* CONFIG_MODULES */
-
+	release_probes(old);
 
-/*
- * Update probes, removing the faulty probes.
- * Called with tracepoints_mutex held.
- */
-static void tracepoint_update_probes(void)
-{
-	/* Core kernel tracepoints */
-	tracepoint_update_probe_range(__start___tracepoints_ptrs,
-		__stop___tracepoints_ptrs);
-	/* tracepoints in modules. */
-	module_update_tracepoints();
-}
+	if (!tp_funcs) {
+		/* Removed last function */
+		if (tp->unregfunc && static_key_enabled(&tp->key))
+			tp->unregfunc();
 
-static struct tracepoint_func *
-tracepoint_add_probe(const char *name, void *probe, void *data)
-{
-	struct tracepoint_entry *entry;
-	struct tracepoint_func *old;
-
-	entry = get_tracepoint(name);
-	if (!entry) {
-		entry = add_tracepoint(name);
-		if (IS_ERR(entry))
-			return (struct tracepoint_func *)entry;
+		if (static_key_enabled(&tp->key))
+			static_key_slow_dec(&tp->key);
 	}
-	old = tracepoint_entry_add_probe(entry, probe, data);
-	if (IS_ERR(old) && !entry->refcount)
-		remove_tracepoint(entry);
-	return old;
+	rcu_assign_pointer(tp->funcs, tp_funcs);
+	return 0;
 }
 
 /**
  * tracepoint_probe_register -  Connect a probe to a tracepoint
- * @name: tracepoint name
+ * @tp: tracepoint
  * @probe: probe handler
  * @data: probe private data
  *
  * Returns 0 if ok, error value on error.
- * The probe address must at least be aligned on the architecture pointer size.
+ * Note: if @tp is within a module, the caller is responsible for
+ * monitoring the module "going" notifier to unregister the probe before
+ * the module is gone.
  */
-int tracepoint_probe_register(const char *name, void *probe, void *data)
+int tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data)
 {
-	struct tracepoint_func *old;
+	struct tracepoint_func tp_func;
+	int ret;
 
 	mutex_lock(&tracepoints_mutex);
-	old = tracepoint_add_probe(name, probe, data);
-	if (IS_ERR(old)) {
-		mutex_unlock(&tracepoints_mutex);
-		return PTR_ERR(old);
-	}
-	tracepoint_update_probes();		/* may update entry */
+	tp_func.func = probe;
+	tp_func.data = data;
+	ret = tracepoint_add_func(tp, &tp_func);
 	mutex_unlock(&tracepoints_mutex);
-	release_probes(old);
-	return 0;
+	return ret;
 }
 EXPORT_SYMBOL_GPL(tracepoint_probe_register);
 
-static struct tracepoint_func *
-tracepoint_remove_probe(const char *name, void *probe, void *data)
-{
-	struct tracepoint_entry *entry;
-	struct tracepoint_func *old;
-
-	entry = get_tracepoint(name);
-	if (!entry)
-		return ERR_PTR(-ENOENT);
-	old = tracepoint_entry_remove_probe(entry, probe, data);
-	if (IS_ERR(old))
-		return old;
-	if (!entry->refcount)
-		remove_tracepoint(entry);
-	return old;
-}
-
 /**
  * tracepoint_probe_unregister -  Disconnect a probe from a tracepoint
- * @name: tracepoint name
+ * @tp: tracepoint
  * @probe: probe function pointer
  * @data: probe private data
  *
- * We do not need to call a synchronize_sched to make sure the probes have
- * finished running before doing a module unload, because the module unload
- * itself uses stop_machine(), which insures that every preempt disabled section
- * have finished.
+ * Returns 0 if ok, error value on error.
  */
-int tracepoint_probe_unregister(const char *name, void *probe, void *data)
+int tracepoint_probe_unregister(struct tracepoint *tp, void *probe, void *data)
 {
-	struct tracepoint_func *old;
+	struct tracepoint_func tp_func;
+	int ret;
 
 	mutex_lock(&tracepoints_mutex);
-	old = tracepoint_remove_probe(name, probe, data);
-	if (IS_ERR(old)) {
-		mutex_unlock(&tracepoints_mutex);
-		return PTR_ERR(old);
-	}
-	tracepoint_update_probes();		/* may update entry */
+	tp_func.func = probe;
+	tp_func.data = data;
+	ret = tracepoint_remove_func(tp, &tp_func);
 	mutex_unlock(&tracepoints_mutex);
-	release_probes(old);
-	return 0;
+	return ret;
 }
 EXPORT_SYMBOL_GPL(tracepoint_probe_unregister);
 
-static LIST_HEAD(old_probes);
-static int need_update;
-
-static void tracepoint_add_old_probes(void *old)
-{
-	need_update = 1;
-	if (old) {
-		struct tp_probes *tp_probes = container_of(old,
-			struct tp_probes, probes[0]);
-		list_add(&tp_probes->u.list, &old_probes);
-	}
-}
-
 #ifdef CONFIG_MODULES
 bool trace_module_has_bad_taint(struct module *mod)
 {
 	return mod->taints & ~((1 << TAINT_OOT_MODULE) | (1 << TAINT_CRAP));
 }
 
-static int tracepoint_module_coming(struct module *mod)
+/*
+ * Ensure the tracer unregistered the module's probes before the module
+ * teardown is performed. Prevents leaks of probe and data pointers.
+ */
+static
+void tp_module_going_check_quiescent(struct tracepoint * const *begin,
+		struct tracepoint * const *end)
 {
-	struct tp_module *tp_mod, *iter;
-	int ret = 0;
-
-	/*
-	 * We skip modules that taint the kernel, especially those with different
-	 * module headers (for forced load), to make sure we don't cause a crash.
-	 * Staging and out-of-tree GPL modules are fine.
-	 */
-	if (trace_module_has_bad_taint(mod))
-		return 0;
-	mutex_lock(&tracepoints_mutex);
-	tp_mod = kmalloc(sizeof(struct tp_module), GFP_KERNEL);
-	if (!tp_mod) {
-		ret = -ENOMEM;
-		goto end;
-	}
-	tp_mod->num_tracepoints = mod->num_tracepoints;
-	tp_mod->tracepoints_ptrs = mod->tracepoints_ptrs;
+	struct tracepoint * const *iter;
 
-	/*
-	 * tracepoint_module_list is kept sorted by struct module pointer
-	 * address for iteration on tracepoints from a seq_file that can release
-	 * the mutex between calls.
-	 */
-	list_for_each_entry_reverse(iter, &tracepoint_module_list, list) {
-		BUG_ON(iter == tp_mod);	/* Should never be in the list twice */
-		if (iter < tp_mod) {
-			/* We belong to the location right after iter. */
-			list_add(&tp_mod->list, &iter->list);
-			goto module_added;
-		}
-	}
-	/* We belong to the beginning of the list */
-	list_add(&tp_mod->list, &tracepoint_module_list);
-module_added:
-	tracepoint_update_probe_range(mod->tracepoints_ptrs,
-		mod->tracepoints_ptrs + mod->num_tracepoints);
-end:
-	mutex_unlock(&tracepoints_mutex);
-	return ret;
+	if (!begin)
+		return;
+	for (iter = begin; iter < end; iter++)
+		WARN_ON_ONCE((*iter)->funcs);
 }
 
-static int tracepoint_module_going(struct module *mod)
+static
+int tracepoint_module_going(struct module *mod)
 {
-	struct tp_module *pos;
-
-	mutex_lock(&tracepoints_mutex);
-	tracepoint_update_probe_range(mod->tracepoints_ptrs,
-		mod->tracepoints_ptrs + mod->num_tracepoints);
-	list_for_each_entry(pos, &tracepoint_module_list, list) {
-		if (pos->tracepoints_ptrs == mod->tracepoints_ptrs) {
-			list_del(&pos->list);
-			kfree(pos);
-			break;
-		}
-	}
 	/*
-	 * In the case of modules that were tainted at "coming", we'll simply
-	 * walk through the list without finding it. We cannot use the "tainted"
-	 * flag on "going", in case a module taints the kernel only after being
-	 * loaded.
+	 * We skip modules that taint the kernel, especially those with
+	 * different module headers (for forced load), to make sure we
+	 * don't cause a crash.  Staging and out-of-tree GPL modules are
+	 * fine.
 	 */
-	mutex_unlock(&tracepoints_mutex);
+	if (trace_module_has_bad_taint(mod))
+		return 0;
+	tp_module_going_check_quiescent(mod->tracepoints_ptrs,
+		mod->tracepoints_ptrs + mod->num_tracepoints);
 	return 0;
 }
 
+static
 int tracepoint_module_notify(struct notifier_block *self,
-			     unsigned long val, void *data)
+		unsigned long val, void *data)
 {
 	struct module *mod = data;
 	int ret = 0;
 
 	switch (val) {
 	case MODULE_STATE_COMING:
-		ret = tracepoint_module_coming(mod);
-		break;
 	case MODULE_STATE_LIVE:
 		break;
 	case MODULE_STATE_GOING:
-- 
1.7.10.4

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