net: Fix soft lockups/OOM issues w/ unix garbage collector

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

It has been updated to use wait_event() instead of
wait_event_interruptible().

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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/af_unix.h

···
 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
 

net/unix/af_unix.c

···
 
 	if (NULL == siocb->scm)
 		siocb->scm = &tmp_scm;
+	wait_for_unix_gc();
 	err = scm_send(sock, msg, siocb->scm);
 	if (err < 0)
 		return err;
···
 
 	if (NULL == siocb->scm)
 		siocb->scm = &tmp_scm;
+	wait_for_unix_gc();
 	err = scm_send(sock, msg, siocb->scm);
 	if (err < 0)
 		return err;

net/unix/garbage.c

···
 #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>
···
 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;
 
···
 		list_move_tail(&u->link, &gc_candidates);
 }
 
-/* The external entry point: unix_gc() */
+static bool gc_in_progress = false;
 
+void wait_for_unix_gc(void)
+{
+	wait_event(unix_gc_wait, gc_in_progress == false);
+}
+
+/* The external entry point: unix_gc() */
 void unix_gc(void)
 {
-	static bool gc_in_progress = false;
-
 	struct unix_sock *u;
 	struct unix_sock *next;
 	struct sk_buff_head hitlist;
···
 	/* 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);