[PATCH] net: fix multithreaded signal handling in unix recv routines

From: Rainer Weikusat
Date: Mon Feb 28 2011 - 09:51:17 EST

From: Rainer Weikusat <rweikusat@xxxxxxxxxxxxxxxxxxxxxxx>

The unix_dgram_recvmsg and unix_stream_recvmsg routines in
net/af_unix.c utilize mutex_lock(&u->readlock) calls in order to
serialize read operations of multiple threads on a single socket. This
implies that, if all n threads of a process block in an AF_UNIX recv
call trying to read data from the same socket, one of these threads
will be sleeping in state TASK_INTERRUPTIBLE and all others in state
TASK_UNINTERRUPTIBLE. Provided that a particular signal is supposed to
be handled by a signal handler defined by the process and that none of
this threads is blocking the signal, the complete_signal routine in
kernel/signal.c will select the 'first' such thread it happens to
encounter when deciding which thread to notify that a signal is
supposed to be handled and if this is one of the TASK_UNINTERRUPTIBLE
threads, the signal won't be handled until the one thread not blocking
on the u->readlock mutex is woken up because some data to process has
arrived (if this ever happens). The included patch fixes this by
changing mutex_lock to mutex_lock_interruptible and handling possible
error returns in the same way interruptions are handled by the actual

Signed-off-by: Rainer Weikusat <rweikusat@xxxxxxxxxxxxxxxxxxxxxxx>

diff -urp net-2.6/net/unix/af_unix.c net-2.6-patched//net/unix/af_unix.c
--- net-2.6/net/unix/af_unix.c 2011-02-16 22:19:43.338358559 +0000
+++ net-2.6-patched//net/unix/af_unix.c 2011-02-16 22:38:39.483543598 +0000
@@ -1724,7 +1724,11 @@ static int unix_dgram_recvmsg(struct kio

msg->msg_namelen = 0;

- mutex_lock(&u->readlock);
+ err = mutex_lock_interruptible(&u->readlock);
+ if (err) {
+ err = sock_intr_errno(sock_rcvtimeo(sk, noblock));
+ goto out;
+ }

skb = skb_recv_datagram(sk, flags, noblock, &err);
if (!skb) {
@@ -1864,7 +1868,11 @@ static int unix_stream_recvmsg(struct ki
memset(&tmp_scm, 0, sizeof(tmp_scm));

- mutex_lock(&u->readlock);
+ err = mutex_lock_interruptible(&u->readlock);
+ if (err) {
+ err = sock_intr_errno(timeo);
+ goto out;
+ }

do {
int chunk;
@@ -1895,11 +1903,12 @@ static int unix_stream_recvmsg(struct ki

timeo = unix_stream_data_wait(sk, timeo);

- if (signal_pending(current)) {
+ if (signal_pending(current)
+ || mutex_lock_interruptible(&u->readlock)) {
err = sock_intr_errno(timeo);
goto out;
- mutex_lock(&u->readlock);
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