[PATCH] dcache: avoid RCU for never-hashed dentries

Some dentries don't need to be globally visible in dentry hashtable.
(pipes & sockets)

Such dentries dont need to wait for a RCU grace period at delete time.
Being able to free them permits a better CPU cache use (hot cache)

This patch combined with (dont insert pipe dentries into dentry_hashtable)
reduced time of { pipe(p); close(p[0]); close(p[1]);} on my UP machine (1.6
GHz Pentium-M) from 3.23 us to 2.86 us (But this patch does not depend on
other patches, only bench results)
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Maneesh Soni <maneesh@in.ibm.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Acked-by: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/dcache.c

@@ -68,15 +68,19 @@ struct dentry_stat_t dentry_stat = {
 	.age_limit = 45,
 };
 
-static void d_callback(struct rcu_head *head)
+static void __d_free(struct dentry *dentry)
 {
-	struct dentry * dentry = container_of(head, struct dentry, d_u.d_rcu);
-
 	if (dname_external(dentry))
 		kfree(dentry->d_name.name);
 	kmem_cache_free(dentry_cache, dentry); 
 }
 
+static void d_callback(struct rcu_head *head)
+{
+	struct dentry * dentry = container_of(head, struct dentry, d_u.d_rcu);
+	__d_free(dentry);
+}
+
 /*
  * no dcache_lock, please.  The caller must decrement dentry_stat.nr_dentry
  * inside dcache_lock.
@@ -85,7 +89,11 @@ static void d_free(struct dentry *dentry)
 {
 	if (dentry->d_op && dentry->d_op->d_release)
 		dentry->d_op->d_release(dentry);
- 	call_rcu(&dentry->d_u.d_rcu, d_callback);
+	/* if dentry was never inserted into hash, immediate free is OK */
+	if (dentry->d_hash.pprev == NULL)
+		__d_free(dentry);
+	else
+		call_rcu(&dentry->d_u.d_rcu, d_callback);
 }
 
 /*