drbd: report congestion if we are waiting for some userland callback

If the drbd worker thread is synchronously waiting for some userland
callback, we don't want some casual pageout to block on us.
Have drbd_congested() report congestion in that case.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_int.h

@@ -815,6 +815,12 @@ enum {
 	CONN_DRY_RUN,		/* Expect disconnect after resync handshake. */
 	CREATE_BARRIER,		/* next P_DATA is preceded by a P_BARRIER */
 	STATE_SENT,		/* Do not change state/UUIDs while this is set */
+	CALLBACK_PENDING,	/* Whether we have a call_usermodehelper(, UMH_WAIT_PROC)
+				 * pending, from drbd worker context.
+				 * If set, bdi_write_congested() returns true,
+				 * so shrink_page_list() would not recurse into,
+				 * and potentially deadlock on, this drbd worker.
+				 */
 };
 
 struct drbd_tconn {			/* is a resource from the config file */

drivers/block/drbd/drbd_main.c

@@ -2338,6 +2338,22 @@ static int drbd_congested(void *congested_data, int bdi_bits)
 		goto out;
 	}
 
+	if (test_bit(CALLBACK_PENDING, &mdev->tconn->flags)) {
+		r |= (1 << BDI_async_congested);
+		/* Without good local data, we would need to read from remote,
+		 * and that would need the worker thread as well, which is
+		 * currently blocked waiting for that usermode helper to
+		 * finish.
+		 */
+		if (!get_ldev_if_state(mdev, D_UP_TO_DATE))
+			r |= (1 << BDI_sync_congested);
+		else
+			put_ldev(mdev);
+		r &= bdi_bits;
+		reason = 'c';
+		goto out;
+	}
+
 	if (get_ldev(mdev)) {
 		q = bdev_get_queue(mdev->ldev->backing_bdev);
 		r = bdi_congested(&q->backing_dev_info, bdi_bits);

drivers/block/drbd/drbd_nl.c

@@ -323,11 +323,15 @@ int drbd_khelper(struct drbd_conf *mdev, char *cmd)
 			NULL };
 	char mb[12];
 	char *argv[] = {usermode_helper, cmd, mb, NULL };
+	struct drbd_tconn *tconn = mdev->tconn;
 	struct sib_info sib;
 	int ret;
 
+	if (current == tconn->worker.task)
+		set_bit(CALLBACK_PENDING, &tconn->flags);
+
 	snprintf(mb, 12, "minor-%d", mdev_to_minor(mdev));
-	setup_khelper_env(mdev->tconn, envp);
+	setup_khelper_env(tconn, envp);
 
 	/* The helper may take some time.
 	 * write out any unsynced meta data changes now */
@@ -350,6 +354,9 @@ int drbd_khelper(struct drbd_conf *mdev, char *cmd)
 	sib.helper_exit_code = ret;
 	drbd_bcast_event(mdev, &sib);
 
+	if (current == tconn->worker.task)
+		clear_bit(CALLBACK_PENDING, &tconn->flags);
+
 	if (ret < 0) /* Ignore any ERRNOs we got. */
 		ret = 0;