net: fix lock_sock_bh/unlock_sock_bh

This new sock lock primitive was introduced to speedup some user context
socket manipulation. But it is unsafe to protect two threads, one using
regular lock_sock/release_sock, one using lock_sock_bh/unlock_sock_bh

This patch changes lock_sock_bh to be careful against 'owned' state.
If owned is found to be set, we must take the slow path.
lock_sock_bh() now returns a boolean to say if the slow path was taken,
and this boolean is used at unlock_sock_bh time to call the appropriate
unlock function.

After this change, BH are either disabled or enabled during the
lock_sock_bh/unlock_sock_bh protected section. This might be misleading,
so we rename these functions to lock_sock_fast()/unlock_sock_fast().

Reported-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Tested-by: Anton Blanchard <anton@samba.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 37 insertions, 2 deletions

diff --git a/net/core/datagram.c b/net/core/datagram.c
index e0097531417a..f5b6f43a4c2e 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -229,15 +229,17 @@ EXPORT_SYMBOL(skb_free_datagram);
 
 void skb_free_datagram_locked(struct sock *sk, struct sk_buff *skb)
 {
+        bool slow;
+
         if (likely(atomic_read(&skb->users) == 1))
                 smp_rmb();
         else if (likely(!atomic_dec_and_test(&skb->users)))
                 return;
 
-        lock_sock_bh(sk);
+        slow = lock_sock_fast(sk);
         skb_orphan(skb);
         sk_mem_reclaim_partial(sk);
-        unlock_sock_bh(sk);
+        unlock_sock_fast(sk, slow);
 
         /* skb is now orphaned, can be freed outside of locked section */
         __kfree_skb(skb);
diff --git a/net/core/sock.c b/net/core/sock.c
index 37fe9b6adade..2cf7f9f7e775 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2007,6 +2007,39 @@ void release_sock(struct sock *sk)
 }
 EXPORT_SYMBOL(release_sock);
 
+/**
+ * lock_sock_fast - fast version of lock_sock
+ * @sk: socket
+ *
+ * This version should be used for very small section, where process wont block
+ * return false if fast path is taken
+ *   sk_lock.slock locked, owned = 0, BH disabled
+ * return true if slow path is taken
+ *   sk_lock.slock unlocked, owned = 1, BH enabled
+ */
+bool lock_sock_fast(struct sock *sk)
+{
+        might_sleep();
+        spin_lock_bh(&sk->sk_lock.slock);
+
+        if (!sk->sk_lock.owned)
+                /*
+                 * Note : We must disable BH
+                 */
+                return false;
+
+        __lock_sock(sk);
+        sk->sk_lock.owned = 1;
+        spin_unlock(&sk->sk_lock.slock);
+        /*
+         * The sk_lock has mutex_lock() semantics here:
+         */
+        mutex_acquire(&sk->sk_lock.dep_map, 0, 0, _RET_IP_);
+        local_bh_enable();
+        return true;
+}
+EXPORT_SYMBOL(lock_sock_fast);
+
 int sock_get_timestamp(struct sock *sk, struct timeval __user *userstamp)
 {
         struct timeval tv;