path: root/net
diff options
authorRainer Weikusat <rweikusat@mobileactivedefense.com>2015-12-06 21:11:38 +0000
committerDavid S. Miller <davem@davemloft.net>2015-12-06 23:31:54 -0500
commit64874280889e7c0b2c9266705363627d4c92cf01 (patch)
tree2e7d955b8093dbd3cfc80a68ae1e4c648a34e74f /net
parentcore: enable more fine-grained datagram reception control (diff)
af_unix: fix unix_dgram_recvmsg entry locking
The current unix_dgram_recvsmg code acquires the u->readlock mutex in order to protect access to the peek offset prior to calling __skb_recv_datagram for actually receiving data. This implies that a blocking reader will go to sleep with this mutex held if there's presently no data to return to userspace. Two non-desirable side effects of this are that a later non-blocking read call on the same socket will block on the ->readlock mutex until the earlier blocking call releases it (or the readers is interrupted) and that later blocking read calls will wait longer than the effective socket read timeout says they should: The timeout will only start 'ticking' once such a reader hits the schedule_timeout in wait_for_more_packets (core.c) while the time it already had to wait until it could acquire the mutex is unaccounted for. The patch avoids both by using the __skb_try_recv_datagram and __skb_wait_for_more packets functions created by the first patch to implement a unix_dgram_recvmsg read loop which releases the readlock mutex prior to going to sleep and reacquires it as needed afterwards. Non-blocking readers will thus immediately return with -EAGAIN if there's no data available regardless of any concurrent blocking readers and all blocking readers will end up sleeping via schedule_timeout, thus honouring the configured socket receive timeout. Signed-off-by: Rainer Weikusat <rweikusat@mobileactivedefense.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net')
1 files changed, 20 insertions, 15 deletions
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 502e572af3fd..1c3c1f3a3ec4 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2078,8 +2078,8 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg,
struct scm_cookie scm;
struct sock *sk = sock->sk;
struct unix_sock *u = unix_sk(sk);
- int noblock = flags & MSG_DONTWAIT;
- struct sk_buff *skb;
+ struct sk_buff *skb, *last;
+ long timeo;
int err;
int peeked, skip;
@@ -2087,26 +2087,32 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg,
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;
- }
+ timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT);
- skip = sk_peek_offset(sk, flags);
+ do {
+ mutex_lock(&u->readlock);
- skb = __skb_recv_datagram(sk, flags, &peeked, &skip, &err);
- if (!skb) {
+ skip = sk_peek_offset(sk, flags);
+ skb = __skb_try_recv_datagram(sk, flags, &peeked, &skip, &err,
+ &last);
+ if (skb)
+ break;
+ mutex_unlock(&u->readlock);
+ if (err != -EAGAIN)
+ break;
+ } while (timeo &&
+ !__skb_wait_for_more_packets(sk, &err, &timeo, last));
+ if (!skb) { /* implies readlock unlocked */
/* Signal EOF on disconnected non-blocking SEQPACKET socket. */
if (sk->sk_type == SOCK_SEQPACKET && err == -EAGAIN &&
(sk->sk_shutdown & RCV_SHUTDOWN))
err = 0;
- goto out_unlock;
+ goto out;
if (wq_has_sleeper(&u->peer_wait))
@@ -2164,7 +2170,6 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg,
skb_free_datagram(sk, skb);
return err;