md: avoid oops on unload if some process is in poll or select.

If md-mod is unloaded while some process is in poll() or select(),
then that process maintains a pointer to md_event_waiters, and when
the try to unlink from that list, they will oops.

The procfs infrastructure ensures that ->poll won't be called after
remove_proc_entry, but doesn't provide a wait_queue_head for us to
use, and the waitqueue code doesn't provide a way to remove all
listeners from a waitqueue.

So we need to:
 1/ make sure no further references to md_event_waiters are taken (by
    setting md_unloading)
 2/ wake up all processes currently waiting, and
 3/ wait until all those processes have disconnected from our
    wait_queue_head.
Reported-by: "majianpeng" <majianpeng@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -7165,11 +7165,14 @@ static int md_seq_open(struct inode *inode, struct file *file)
 	return error;
 }
 
+static int md_unloading;
 static unsigned int mdstat_poll(struct file *filp, poll_table *wait)
 {
 	struct seq_file *seq = filp->private_data;
 	int mask;
 
+	if (md_unloading)
+		return POLLIN|POLLRDNORM|POLLERR|POLLPRI;;
 	poll_wait(filp, &md_event_waiters, wait);
 
 	/* always allow read */
@@ -8655,6 +8658,7 @@ static __exit void md_exit(void)
 {
 	struct mddev *mddev;
 	struct list_head *tmp;
+	int delay = 1;
 
 	blk_unregister_region(MKDEV(MD_MAJOR,0), 1U << MINORBITS);
 	blk_unregister_region(MKDEV(mdp_major,0), 1U << MINORBITS);
@@ -8663,7 +8667,19 @@ static __exit void md_exit(void)
 	unregister_blkdev(mdp_major, "mdp");
 	unregister_reboot_notifier(&md_notifier);
 	unregister_sysctl_table(raid_table_header);
+
+	/* We cannot unload the modules while some process is
+	 * waiting for us in select() or poll() - wake them up
+	 */
+	md_unloading = 1;
+	while (waitqueue_active(&md_event_waiters)) {
+		/* not safe to leave yet */
+		wake_up(&md_event_waiters);
+		msleep(delay);
+		delay += delay;
+	}
 	remove_proc_entry("mdstat", NULL);
+
 	for_each_mddev(mddev, tmp) {
 		export_array(mddev);
 		mddev->hold_active = 0;