[PATCH] [DCACHE] : avoid RCU for never hashed dentries

[Eric Dumazet]

Some dentries dont 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@xxxxxxxxxxxxx>
--- linux-2.6.19-rc6/fs/dcache.c	2006-11-22 17:33:33.000000000 +0100
+++ linux-2.6.19-rc6-ed/fs/dcache.c	2006-11-22 17:33:58.000000000 +0100
@@ -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);
 }
 
 /*