tracepoint: simplification for tracepoints using RCU

Impact: simplify implementation

Now, unused memory is handled by struct tp_probes.

old code use these three field to handle unused memory.
struct tracepoint_entry {
	...
	struct rcu_head rcu;
	void *oldptr;
	unsigned char rcu_pending:1;
	...
};

in this way, unused memory is handled by struct tracepoint_entry.
it bring reenter bug(it was fixed) and tracepoint.c is filled
full of ".*rcu.*" code statements. this patch removes all these.

and:
  rcu_barrier_sched() is removed.
  Do not need regain tracepoints_mutex after tracepoint_update_probes()
  several little cleanup.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index af8c856..3e22867 100644 (file)
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -43,6 +43,7 @@ static DEFINE_MUTEX(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 :
@@ -54,40 +55,40 @@ struct tracepoint_entry {
        struct hlist_node hlist;
        void **funcs;
        int refcount;   /* Number of times armed. 0 if disarmed. */
-       struct rcu_head rcu;
-       void *oldptr;
-       unsigned char rcu_pending:1;
        char name[0];
 };
 
-static struct hlist_head tracepoint_table[TRACEPOINT_TABLE_SIZE];
+struct tp_probes {
+       struct rcu_head rcu;
+       void *probes[0];
+};
 
-static void free_old_closure(struct rcu_head *head)
+static inline void *allocate_probes(int count)
 {
-       struct tracepoint_entry *entry = container_of(head,
-               struct tracepoint_entry, rcu);
-       kfree(entry->oldptr);
-       /* Make sure we free the data before setting the pending flag to 0 */
-       smp_wmb();
-       entry->rcu_pending = 0;
+       struct tp_probes *p  = kmalloc(count * sizeof(void *)
+                       + sizeof(struct tp_probes), GFP_KERNEL);
+       return p == NULL ? NULL : p->probes;
 }
 
-static void tracepoint_entry_free_old(struct tracepoint_entry *entry, void *old)
+static void rcu_free_old_probes(struct rcu_head *head)
 {
-       if (!old)
-               return;
-       entry->oldptr = old;
-       entry->rcu_pending = 1;
-       /* write rcu_pending before calling the RCU callback */
-       smp_wmb();
-       call_rcu_sched(&entry->rcu, free_old_closure);
+       kfree(container_of(head, struct tp_probes, rcu));
+}
+
+static inline void release_probes(void *old)
+{
+       if (old) {
+               struct tp_probes *tp_probes = container_of(old,
+                       struct tp_probes, probes[0]);
+               call_rcu(&tp_probes->rcu, rcu_free_old_probes);
+       }
 }
 
 static void debug_print_probes(struct tracepoint_entry *entry)
 {
        int i;
 
-       if (!tracepoint_debug)
+       if (!tracepoint_debug || !entry->funcs)
                return;
 
        for (i = 0; entry->funcs[i]; i++)
@@ -111,12 +112,13 @@ tracepoint_entry_add_probe(struct tracepoint_entry *entry, void *probe)
                                return ERR_PTR(-EEXIST);
        }
        /* + 2 : one for new probe, one for NULL func */
-       new = kzalloc((nr_probes + 2) * sizeof(void *), GFP_KERNEL);
+       new = allocate_probes(nr_probes + 2);
        if (new == NULL)
                return ERR_PTR(-ENOMEM);
        if (old)
                memcpy(new, old, nr_probes * sizeof(void *));
        new[nr_probes] = probe;
+       new[nr_probes + 1] = NULL;
        entry->refcount = nr_probes + 1;
        entry->funcs = new;
        debug_print_probes(entry);
@@ -132,7 +134,7 @@ tracepoint_entry_remove_probe(struct tracepoint_entry *entry, void *probe)
        old = entry->funcs;
 
        if (!old)
-               return NULL;
+               return ERR_PTR(-ENOENT);
 
        debug_print_probes(entry);
        /* (N -> M), (N > 1, M >= 0) probes */
@@ -151,13 +153,13 @@ tracepoint_entry_remove_probe(struct tracepoint_entry *entry, void *probe)
                int j = 0;
                /* N -> M, (N > 1, M > 0) */
                /* + 1 for NULL */
-               new = kzalloc((nr_probes - nr_del + 1)
-                       * sizeof(void *), GFP_KERNEL);
+               new = allocate_probes(nr_probes - nr_del + 1);
                if (new == NULL)
                        return ERR_PTR(-ENOMEM);
                for (i = 0; old[i]; i++)
                        if ((probe && old[i] != probe))
                                new[j++] = old[i];
+               new[nr_probes - nr_del] = NULL;
                entry->refcount = nr_probes - nr_del;
                entry->funcs = new;
        }
@@ -215,7 +217,6 @@ static struct tracepoint_entry *add_tracepoint(const char *name)
        memcpy(&e->name[0], name, name_len);
        e->funcs = NULL;
        e->refcount = 0;
-       e->rcu_pending = 0;
        hlist_add_head(&e->hlist, head);
        return e;
 }
@@ -224,32 +225,10 @@ static struct tracepoint_entry *add_tracepoint(const char *name)
  * Remove the tracepoint from the tracepoint hash table. Must be called with
  * mutex_lock held.
  */
-static int remove_tracepoint(const char *name)
+static inline void remove_tracepoint(struct tracepoint_entry *e)
 {
-       struct hlist_head *head;
-       struct hlist_node *node;
-       struct tracepoint_entry *e;
-       int found = 0;
-       size_t len = strlen(name) + 1;
-       u32 hash = jhash(name, len-1, 0);
-
-       head = &tracepoint_table[hash & (TRACEPOINT_TABLE_SIZE - 1)];
-       hlist_for_each_entry(e, node, head, hlist) {
-               if (!strcmp(name, e->name)) {
-                       found = 1;
-                       break;
-               }
-       }
-       if (!found)
-               return -ENOENT;
-       if (e->refcount)
-               return -EBUSY;
        hlist_del(&e->hlist);
-       /* Make sure the call_rcu_sched has been executed */
-       if (e->rcu_pending)
-               rcu_barrier_sched();
        kfree(e);
-       return 0;
 }
 
 /*
@@ -343,25 +322,17 @@ int tracepoint_probe_register(const char *name, void *probe)
                        goto end;
                }
        }
-       /*
-        * If we detect that a call_rcu_sched is pending for this tracepoint,
-        * make sure it's executed now.
-        */
-       if (entry->rcu_pending)
-               rcu_barrier_sched();
        old = tracepoint_entry_add_probe(entry, probe);
        if (IS_ERR(old)) {
+               if (!entry->refcount)
+                       remove_tracepoint(entry);
                ret = PTR_ERR(old);
                goto end;
        }
        mutex_unlock(&tracepoints_mutex);
        tracepoint_update_probes();             /* may update entry */
-       mutex_lock(&tracepoints_mutex);
-       entry = get_tracepoint(name);
-       WARN_ON(!entry);
-       if (entry->rcu_pending)
-               rcu_barrier_sched();
-       tracepoint_entry_free_old(entry, old);
+       release_probes(old);
+       return 0;
 end:
        mutex_unlock(&tracepoints_mutex);
        return ret;
@@ -388,25 +359,17 @@ int tracepoint_probe_unregister(const char *name, void *probe)
        entry = get_tracepoint(name);
        if (!entry)
                goto end;
-       if (entry->rcu_pending)
-               rcu_barrier_sched();
        old = tracepoint_entry_remove_probe(entry, probe);
-       if (!old) {
-               printk(KERN_WARNING "Warning: Trying to unregister a probe"
-                                   "that doesn't exist\n");
+       if (IS_ERR(old)) {
+               ret = PTR_ERR(old);
                goto end;
        }
+       if (!entry->refcount)
+               remove_tracepoint(entry);
        mutex_unlock(&tracepoints_mutex);
        tracepoint_update_probes();             /* may update entry */
-       mutex_lock(&tracepoints_mutex);
-       entry = get_tracepoint(name);
-       if (!entry)
-               goto end;
-       if (entry->rcu_pending)
-               rcu_barrier_sched();
-       tracepoint_entry_free_old(entry, old);
-       remove_tracepoint(name);        /* Ignore busy error message */
-       ret = 0;
+       release_probes(old);
+       return 0;
 end:
        mutex_unlock(&tracepoints_mutex);
        return ret;