linux-kernel - [PATCH 3.2 74/77] af_unix: Revert 'lock_interruptible' in stream receive code

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Date:	Thu, 24 Dec 2015 15:37:42 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "David S. Miller" <davem@...emloft.net>,
	"Rainer Weikusat" <rweikusat@...ileactivedefense.com>,
	"Hannes Frederic Sowa" <hannes@...essinduktion.org>
Subject: [PATCH 3.2 74/77] af_unix: Revert 'lock_interruptible' in stream
 receive  code

3.2.75-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Rainer Weikusat <rweikusat@...ileactivedefense.com>

[ Upstream commit 3822b5c2fc62e3de8a0f33806ff279fb7df92432 ]

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@...ileactivedefense.com>
Acked-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 net/unix/af_unix.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1913,14 +1913,7 @@ static int unix_dgram_recvmsg(struct kio
 	if (flags&MSG_OOB)
 		goto out;
 
-	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);
 
 	skb = skb_recv_datagram(sk, flags, noblock, &err);
 	if (!skb) {
@@ -2105,12 +2098,12 @@ static int unix_stream_recvmsg(struct ki
 
 			timeo = unix_stream_data_wait(sk, timeo);
 
-			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);

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