[PATCH] Fix soft lockups/OOM issues w/ unix garbage collector

[dann frazier <dannf@hp.com>]

This is an implementation of David Miller's suggested fix in:
  https://bugzilla.redhat.com/show_bug.cgi?id=470201

Paraphrasing the description from the above report, it makes sendmsg()
block while UNIX garbage collection is in progress. This avoids a
situation where child processes continue to queue new FDs over a
AF_UNIX socket to a parent which is in the exit path and running
garbage collection on these FDs. This contention can result in soft
lockups and oom-killing of unrelated processes.

Signed-off-by: dann frazier <dannf@hp.com>
---
 include/net/af_unix.h |    1 +
 net/unix/af_unix.c    |    2 ++
 net/unix/garbage.c    |   18 +++++++++++++++---
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index c29ff1d..1614d78 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -9,6 +9,7 @@
 extern void unix_inflight(struct file *fp);
 extern void unix_notinflight(struct file *fp);
 extern void unix_gc(void);
+extern void wait_for_unix_gc(void);
 
 #define UNIX_HASH_SIZE	256
 
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 8bde9bf..b0785ef 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1341,6 +1341,7 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 
 	if (NULL == siocb->scm)
 		siocb->scm = &tmp_scm;
+	wait_for_unix_gc();
 	err = scm_send(sock, msg, siocb->scm);
 	if (err < 0)
 		return err;
@@ -1491,6 +1492,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 
 	if (NULL == siocb->scm)
 		siocb->scm = &tmp_scm;
+	wait_for_unix_gc();
 	err = scm_send(sock, msg, siocb->scm);
 	if (err < 0)
 		return err;
diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 6d4a9a8..cf1b0b0 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -80,6 +80,7 @@
 #include <linux/file.h>
 #include <linux/proc_fs.h>
 #include <linux/mutex.h>
+#include <linux/wait.h>
 
 #include <net/sock.h>
 #include <net/af_unix.h>
@@ -91,6 +92,7 @@
 static LIST_HEAD(gc_inflight_list);
 static LIST_HEAD(gc_candidates);
 static DEFINE_SPINLOCK(unix_gc_lock);
+static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);
 
 unsigned int unix_tot_inflight;
 
@@ -266,12 +268,21 @@ static void inc_inflight_move_tail(struct unix_sock *u)
 		list_move_tail(&u->link, &gc_candidates);
 }
 
-/* The external entry point: unix_gc() */
+static bool gc_in_progress = false;
 
-void unix_gc(void)
+void wait_for_unix_gc(void)
 {
-	static bool gc_in_progress = false;
+	int error;
+
+	do {
+		error = wait_event_interruptible(unix_gc_wait,
+						 gc_in_progress == false);
+	} while(error);
+}
 
+/* The external entry point: unix_gc() */
+void unix_gc(void)
+{
 	struct unix_sock *u;
 	struct unix_sock *next;
 	struct sk_buff_head hitlist;
@@ -376,6 +387,7 @@ void unix_gc(void)
 	/* All candidates should have been detached by now. */
 	BUG_ON(!list_empty(&gc_candidates));
 	gc_in_progress = false;
+	wake_up(&unix_gc_wait);
 
  out:
 	spin_unlock(&unix_gc_lock);