tracepoints: fix reentrancy
[linux-2.6.git] / kernel / tracepoint.c
1 /*
2  * Copyright (C) 2008 Mathieu Desnoyers
3  *
4  * This program is free software; you can redistribute it and/or modify
5  * it under the terms of the GNU General Public License as published by
6  * the Free Software Foundation; either version 2 of the License, or
7  * (at your option) any later version.
8  *
9  * This program is distributed in the hope that it will be useful,
10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
12  * GNU General Public License for more details.
13  *
14  * You should have received a copy of the GNU General Public License
15  * along with this program; if not, write to the Free Software
16  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
17  */
18 #include <linux/module.h>
19 #include <linux/mutex.h>
20 #include <linux/types.h>
21 #include <linux/jhash.h>
22 #include <linux/list.h>
23 #include <linux/rcupdate.h>
24 #include <linux/tracepoint.h>
25 #include <linux/err.h>
26 #include <linux/slab.h>
27
28 extern struct tracepoint __start___tracepoints[];
29 extern struct tracepoint __stop___tracepoints[];
30
31 /* Set to 1 to enable tracepoint debug output */
32 static const int tracepoint_debug;
33
34 /*
35  * tracepoints_mutex nests inside module_mutex. Tracepoints mutex protects the
36  * builtin and module tracepoints and the hash table.
37  */
38 static DEFINE_MUTEX(tracepoints_mutex);
39
40 /*
41  * Tracepoint hash table, containing the active tracepoints.
42  * Protected by tracepoints_mutex.
43  */
44 #define TRACEPOINT_HASH_BITS 6
45 #define TRACEPOINT_TABLE_SIZE (1 << TRACEPOINT_HASH_BITS)
46
47 /*
48  * Note about RCU :
49  * It is used to to delay the free of multiple probes array until a quiescent
50  * state is reached.
51  * Tracepoint entries modifications are protected by the tracepoints_mutex.
52  */
53 struct tracepoint_entry {
54         struct hlist_node hlist;
55         void **funcs;
56         int refcount;   /* Number of times armed. 0 if disarmed. */
57         struct rcu_head rcu;
58         void *oldptr;
59         unsigned char rcu_pending:1;
60         char name[0];
61 };
62
63 static struct hlist_head tracepoint_table[TRACEPOINT_TABLE_SIZE];
64
65 static void free_old_closure(struct rcu_head *head)
66 {
67         struct tracepoint_entry *entry = container_of(head,
68                 struct tracepoint_entry, rcu);
69         kfree(entry->oldptr);
70         /* Make sure we free the data before setting the pending flag to 0 */
71         smp_wmb();
72         entry->rcu_pending = 0;
73 }
74
75 static void tracepoint_entry_free_old(struct tracepoint_entry *entry, void *old)
76 {
77         if (!old)
78                 return;
79         entry->oldptr = old;
80         entry->rcu_pending = 1;
81         /* write rcu_pending before calling the RCU callback */
82         smp_wmb();
83         call_rcu_sched(&entry->rcu, free_old_closure);
84 }
85
86 static void debug_print_probes(struct tracepoint_entry *entry)
87 {
88         int i;
89
90         if (!tracepoint_debug)
91                 return;
92
93         for (i = 0; entry->funcs[i]; i++)
94                 printk(KERN_DEBUG "Probe %d : %p\n", i, entry->funcs[i]);
95 }
96
97 static void *
98 tracepoint_entry_add_probe(struct tracepoint_entry *entry, void *probe)
99 {
100         int nr_probes = 0;
101         void **old, **new;
102
103         WARN_ON(!probe);
104
105         debug_print_probes(entry);
106         old = entry->funcs;
107         if (old) {
108                 /* (N -> N+1), (N != 0, 1) probes */
109                 for (nr_probes = 0; old[nr_probes]; nr_probes++)
110                         if (old[nr_probes] == probe)
111                                 return ERR_PTR(-EEXIST);
112         }
113         /* + 2 : one for new probe, one for NULL func */
114         new = kzalloc((nr_probes + 2) * sizeof(void *), GFP_KERNEL);
115         if (new == NULL)
116                 return ERR_PTR(-ENOMEM);
117         if (old)
118                 memcpy(new, old, nr_probes * sizeof(void *));
119         new[nr_probes] = probe;
120         entry->refcount = nr_probes + 1;
121         entry->funcs = new;
122         debug_print_probes(entry);
123         return old;
124 }
125
126 static void *
127 tracepoint_entry_remove_probe(struct tracepoint_entry *entry, void *probe)
128 {
129         int nr_probes = 0, nr_del = 0, i;
130         void **old, **new;
131
132         old = entry->funcs;
133
134         debug_print_probes(entry);
135         /* (N -> M), (N > 1, M >= 0) probes */
136         for (nr_probes = 0; old[nr_probes]; nr_probes++) {
137                 if ((!probe || old[nr_probes] == probe))
138                         nr_del++;
139         }
140
141         if (nr_probes - nr_del == 0) {
142                 /* N -> 0, (N > 1) */
143                 entry->funcs = NULL;
144                 entry->refcount = 0;
145                 debug_print_probes(entry);
146                 return old;
147         } else {
148                 int j = 0;
149                 /* N -> M, (N > 1, M > 0) */
150                 /* + 1 for NULL */
151                 new = kzalloc((nr_probes - nr_del + 1)
152                         * sizeof(void *), GFP_KERNEL);
153                 if (new == NULL)
154                         return ERR_PTR(-ENOMEM);
155                 for (i = 0; old[i]; i++)
156                         if ((probe && old[i] != probe))
157                                 new[j++] = old[i];
158                 entry->refcount = nr_probes - nr_del;
159                 entry->funcs = new;
160         }
161         debug_print_probes(entry);
162         return old;
163 }
164
165 /*
166  * Get tracepoint if the tracepoint is present in the tracepoint hash table.
167  * Must be called with tracepoints_mutex held.
168  * Returns NULL if not present.
169  */
170 static struct tracepoint_entry *get_tracepoint(const char *name)
171 {
172         struct hlist_head *head;
173         struct hlist_node *node;
174         struct tracepoint_entry *e;
175         u32 hash = jhash(name, strlen(name), 0);
176
177         head = &tracepoint_table[hash & (TRACEPOINT_TABLE_SIZE - 1)];
178         hlist_for_each_entry(e, node, head, hlist) {
179                 if (!strcmp(name, e->name))
180                         return e;
181         }
182         return NULL;
183 }
184
185 /*
186  * Add the tracepoint to the tracepoint hash table. Must be called with
187  * tracepoints_mutex held.
188  */
189 static struct tracepoint_entry *add_tracepoint(const char *name)
190 {
191         struct hlist_head *head;
192         struct hlist_node *node;
193         struct tracepoint_entry *e;
194         size_t name_len = strlen(name) + 1;
195         u32 hash = jhash(name, name_len-1, 0);
196
197         head = &tracepoint_table[hash & (TRACEPOINT_TABLE_SIZE - 1)];
198         hlist_for_each_entry(e, node, head, hlist) {
199                 if (!strcmp(name, e->name)) {
200                         printk(KERN_NOTICE
201                                 "tracepoint %s busy\n", name);
202                         return ERR_PTR(-EEXIST);        /* Already there */
203                 }
204         }
205         /*
206          * Using kmalloc here to allocate a variable length element. Could
207          * cause some memory fragmentation if overused.
208          */
209         e = kmalloc(sizeof(struct tracepoint_entry) + name_len, GFP_KERNEL);
210         if (!e)
211                 return ERR_PTR(-ENOMEM);
212         memcpy(&e->name[0], name, name_len);
213         e->funcs = NULL;
214         e->refcount = 0;
215         e->rcu_pending = 0;
216         hlist_add_head(&e->hlist, head);
217         return e;
218 }
219
220 /*
221  * Remove the tracepoint from the tracepoint hash table. Must be called with
222  * mutex_lock held.
223  */
224 static int remove_tracepoint(const char *name)
225 {
226         struct hlist_head *head;
227         struct hlist_node *node;
228         struct tracepoint_entry *e;
229         int found = 0;
230         size_t len = strlen(name) + 1;
231         u32 hash = jhash(name, len-1, 0);
232
233         head = &tracepoint_table[hash & (TRACEPOINT_TABLE_SIZE - 1)];
234         hlist_for_each_entry(e, node, head, hlist) {
235                 if (!strcmp(name, e->name)) {
236                         found = 1;
237                         break;
238                 }
239         }
240         if (!found)
241                 return -ENOENT;
242         if (e->refcount)
243                 return -EBUSY;
244         hlist_del(&e->hlist);
245         /* Make sure the call_rcu_sched has been executed */
246         if (e->rcu_pending)
247                 rcu_barrier_sched();
248         kfree(e);
249         return 0;
250 }
251
252 /*
253  * Sets the probe callback corresponding to one tracepoint.
254  */
255 static void set_tracepoint(struct tracepoint_entry **entry,
256         struct tracepoint *elem, int active)
257 {
258         WARN_ON(strcmp((*entry)->name, elem->name) != 0);
259
260         /*
261          * rcu_assign_pointer has a smp_wmb() which makes sure that the new
262          * probe callbacks array is consistent before setting a pointer to it.
263          * This array is referenced by __DO_TRACE from
264          * include/linux/tracepoints.h. A matching smp_read_barrier_depends()
265          * is used.
266          */
267         rcu_assign_pointer(elem->funcs, (*entry)->funcs);
268         elem->state = active;
269 }
270
271 /*
272  * Disable a tracepoint and its probe callback.
273  * Note: only waiting an RCU period after setting elem->call to the empty
274  * function insures that the original callback is not used anymore. This insured
275  * by preempt_disable around the call site.
276  */
277 static void disable_tracepoint(struct tracepoint *elem)
278 {
279         elem->state = 0;
280 }
281
282 /**
283  * tracepoint_update_probe_range - Update a probe range
284  * @begin: beginning of the range
285  * @end: end of the range
286  *
287  * Updates the probe callback corresponding to a range of tracepoints.
288  */
289 void tracepoint_update_probe_range(struct tracepoint *begin,
290         struct tracepoint *end)
291 {
292         struct tracepoint *iter;
293         struct tracepoint_entry *mark_entry;
294
295         mutex_lock(&tracepoints_mutex);
296         for (iter = begin; iter < end; iter++) {
297                 mark_entry = get_tracepoint(iter->name);
298                 if (mark_entry) {
299                         set_tracepoint(&mark_entry, iter,
300                                         !!mark_entry->refcount);
301                 } else {
302                         disable_tracepoint(iter);
303                 }
304         }
305         mutex_unlock(&tracepoints_mutex);
306 }
307
308 /*
309  * Update probes, removing the faulty probes.
310  */
311 static void tracepoint_update_probes(void)
312 {
313         /* Core kernel tracepoints */
314         tracepoint_update_probe_range(__start___tracepoints,
315                 __stop___tracepoints);
316         /* tracepoints in modules. */
317         module_update_tracepoints();
318 }
319
320 /**
321  * tracepoint_probe_register -  Connect a probe to a tracepoint
322  * @name: tracepoint name
323  * @probe: probe handler
324  *
325  * Returns 0 if ok, error value on error.
326  * The probe address must at least be aligned on the architecture pointer size.
327  */
328 int tracepoint_probe_register(const char *name, void *probe)
329 {
330         struct tracepoint_entry *entry;
331         int ret = 0;
332         void *old;
333
334         mutex_lock(&tracepoints_mutex);
335         entry = get_tracepoint(name);
336         if (!entry) {
337                 entry = add_tracepoint(name);
338                 if (IS_ERR(entry)) {
339                         ret = PTR_ERR(entry);
340                         goto end;
341                 }
342         }
343         /*
344          * If we detect that a call_rcu_sched is pending for this tracepoint,
345          * make sure it's executed now.
346          */
347         if (entry->rcu_pending)
348                 rcu_barrier_sched();
349         old = tracepoint_entry_add_probe(entry, probe);
350         if (IS_ERR(old)) {
351                 ret = PTR_ERR(old);
352                 goto end;
353         }
354         mutex_unlock(&tracepoints_mutex);
355         tracepoint_update_probes();             /* may update entry */
356         mutex_lock(&tracepoints_mutex);
357         entry = get_tracepoint(name);
358         WARN_ON(!entry);
359         if (entry->rcu_pending)
360                 rcu_barrier_sched();
361         tracepoint_entry_free_old(entry, old);
362 end:
363         mutex_unlock(&tracepoints_mutex);
364         return ret;
365 }
366 EXPORT_SYMBOL_GPL(tracepoint_probe_register);
367
368 /**
369  * tracepoint_probe_unregister -  Disconnect a probe from a tracepoint
370  * @name: tracepoint name
371  * @probe: probe function pointer
372  *
373  * We do not need to call a synchronize_sched to make sure the probes have
374  * finished running before doing a module unload, because the module unload
375  * itself uses stop_machine(), which insures that every preempt disabled section
376  * have finished.
377  */
378 int tracepoint_probe_unregister(const char *name, void *probe)
379 {
380         struct tracepoint_entry *entry;
381         void *old;
382         int ret = -ENOENT;
383
384         mutex_lock(&tracepoints_mutex);
385         entry = get_tracepoint(name);
386         if (!entry)
387                 goto end;
388         if (entry->rcu_pending)
389                 rcu_barrier_sched();
390         old = tracepoint_entry_remove_probe(entry, probe);
391         mutex_unlock(&tracepoints_mutex);
392         tracepoint_update_probes();             /* may update entry */
393         mutex_lock(&tracepoints_mutex);
394         entry = get_tracepoint(name);
395         if (!entry)
396                 goto end;
397         if (entry->rcu_pending)
398                 rcu_barrier_sched();
399         tracepoint_entry_free_old(entry, old);
400         remove_tracepoint(name);        /* Ignore busy error message */
401         ret = 0;
402 end:
403         mutex_unlock(&tracepoints_mutex);
404         return ret;
405 }
406 EXPORT_SYMBOL_GPL(tracepoint_probe_unregister);
407
408 /**
409  * tracepoint_get_iter_range - Get a next tracepoint iterator given a range.
410  * @tracepoint: current tracepoints (in), next tracepoint (out)
411  * @begin: beginning of the range
412  * @end: end of the range
413  *
414  * Returns whether a next tracepoint has been found (1) or not (0).
415  * Will return the first tracepoint in the range if the input tracepoint is
416  * NULL.
417  */
418 int tracepoint_get_iter_range(struct tracepoint **tracepoint,
419         struct tracepoint *begin, struct tracepoint *end)
420 {
421         if (!*tracepoint && begin != end) {
422                 *tracepoint = begin;
423                 return 1;
424         }
425         if (*tracepoint >= begin && *tracepoint < end)
426                 return 1;
427         return 0;
428 }
429 EXPORT_SYMBOL_GPL(tracepoint_get_iter_range);
430
431 static void tracepoint_get_iter(struct tracepoint_iter *iter)
432 {
433         int found = 0;
434
435         /* Core kernel tracepoints */
436         if (!iter->module) {
437                 found = tracepoint_get_iter_range(&iter->tracepoint,
438                                 __start___tracepoints, __stop___tracepoints);
439                 if (found)
440                         goto end;
441         }
442         /* tracepoints in modules. */
443         found = module_get_iter_tracepoints(iter);
444 end:
445         if (!found)
446                 tracepoint_iter_reset(iter);
447 }
448
449 void tracepoint_iter_start(struct tracepoint_iter *iter)
450 {
451         tracepoint_get_iter(iter);
452 }
453 EXPORT_SYMBOL_GPL(tracepoint_iter_start);
454
455 void tracepoint_iter_next(struct tracepoint_iter *iter)
456 {
457         iter->tracepoint++;
458         /*
459          * iter->tracepoint may be invalid because we blindly incremented it.
460          * Make sure it is valid by marshalling on the tracepoints, getting the
461          * tracepoints from following modules if necessary.
462          */
463         tracepoint_get_iter(iter);
464 }
465 EXPORT_SYMBOL_GPL(tracepoint_iter_next);
466
467 void tracepoint_iter_stop(struct tracepoint_iter *iter)
468 {
469 }
470 EXPORT_SYMBOL_GPL(tracepoint_iter_stop);
471
472 void tracepoint_iter_reset(struct tracepoint_iter *iter)
473 {
474         iter->module = NULL;
475         iter->tracepoint = NULL;
476 }
477 EXPORT_SYMBOL_GPL(tracepoint_iter_reset);