[PATCH 3.16.y-ckt 099/126] af_unix: Revert 'lock_interruptible' in stream receive code

[Luis Henriques]

3.16.7-ckt22 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Rainer Weikusat <rweikusat at mobileactivedefense.com>

commit 3822b5c2fc62e3de8a0f33806ff279fb7df92432 upstream.

With b3ca9b02b00704053a38bfe4c31dbbb9c13595d0, the AF_UNIX SOCK_STREAM
receive code was changed from using mutex_lock(&u->readlock) to
mutex_lock_interruptible(&u->readlock) to prevent signals from being
delayed for an indefinite time if a thread sleeping on the mutex
happened to be selected for handling the signal. But this was never a
problem with the stream receive code (as opposed to its datagram
counterpart) as that never went to sleep waiting for new messages with the
mutex held and thus, wouldn't cause secondary readers to block on the
mutex waiting for the sleeping primary reader. As the interruptible
locking makes the code more complicated in exchange for no benefit,
change it back to using mutex_lock.

Signed-off-by: Rainer Weikusat <rweikusat at mobileactivedefense.com>
Acked-by: Hannes Frederic Sowa <hannes at stressinduktion.org>
Signed-off-by: David S. Miller <davem at davemloft.net>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 net/unix/af_unix.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 9acfd79f0d13..7229794c1419 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2096,14 +2096,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 		memset(&tmp_scm, 0, sizeof(tmp_scm));
 	}
 
-	err = mutex_lock_interruptible(&u->readlock);
-	if (unlikely(err)) {
-		/* recvmsg() in non blocking mode is supposed to return -EAGAIN
-		 * sk_rcvtimeo is not honored by mutex_lock_interruptible()
-		 */
-		err = noblock ? -EAGAIN : -ERESTARTSYS;
-		goto out;
-	}
+	mutex_lock(&u->readlock);
 
 	if (flags & MSG_PEEK)
 		skip = sk_peek_offset(sk, flags);
@@ -2144,12 +2137,12 @@ again:
 
 			timeo = unix_stream_data_wait(sk, timeo, last);
 
-			if (signal_pending(current)
-			    ||  mutex_lock_interruptible(&u->readlock)) {
+			if (signal_pending(current)) {
 				err = sock_intr_errno(timeo);
 				goto out;
 			}
 
+			mutex_lock(&u->readlock);
 			continue;
  unlock:
 			unix_state_unlock(sk);