[AF_UNIX]: Convert to use a spinlock instead of rwlock

From: Benjamin LaHaise <bcrl@kvack.org>

In af_unix, a rwlock is used to protect internal state.  At least on my 
P4 with HT it is faster to use a spinlock due to the simpler memory 
barrier used to unlock.  This patch raises bw_unix to ~690K/s.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/af_unix.h

@@ -58,10 +58,10 @@ struct unix_skb_parms {
 #define UNIXCB(skb) 	(*(struct unix_skb_parms*)&((skb)->cb))
 #define UNIXCREDS(skb)	(&UNIXCB((skb)).creds)
 
-#define unix_state_rlock(s)	read_lock(&unix_sk(s)->lock)
-#define unix_state_runlock(s)	read_unlock(&unix_sk(s)->lock)
-#define unix_state_wlock(s)	write_lock(&unix_sk(s)->lock)
-#define unix_state_wunlock(s)	write_unlock(&unix_sk(s)->lock)
+#define unix_state_rlock(s)	spin_lock(&unix_sk(s)->lock)
+#define unix_state_runlock(s)	spin_unlock(&unix_sk(s)->lock)
+#define unix_state_wlock(s)	spin_lock(&unix_sk(s)->lock)
+#define unix_state_wunlock(s)	spin_unlock(&unix_sk(s)->lock)
 
 #ifdef __KERNEL__
 /* The AF_UNIX socket */
@@ -76,7 +76,7 @@ struct unix_sock {
         struct sock		*other;
         struct sock		*gc_tree;
         atomic_t                inflight;
-        rwlock_t                lock;
+        spinlock_t		lock;
         wait_queue_head_t       peer_wait;
 };
 #define unix_sk(__sk) ((struct unix_sock *)__sk)

net/unix/af_unix.c

@@ -564,7 +564,7 @@ static struct sock * unix_create1(struct socket *sock)
 	u	  = unix_sk(sk);
 	u->dentry = NULL;
 	u->mnt	  = NULL;
-	rwlock_init(&u->lock);
+	spin_lock_init(&u->lock);
 	atomic_set(&u->inflight, sock ? 0 : -1);
 	init_MUTEX(&u->readsem); /* single task reading lock */
 	init_waitqueue_head(&u->peer_wait);