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>

1 files changed, 37 insertions, 74 deletions

diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index af8c85664882..3e22867184e3 100644
--- 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;