drbd: fix race on meta-data update, addendum

addendum to baa33ae4eaa4477b60af7c434c0ddd1d182c1ae7

The race:
    drbd_md_sync()
	if (!test_and_clear_bit(MD_DIRTY, &mdev->flags))
		return;
    ==> RACE with drbd_md_mark_dirty() rearming the timer.
	del_timer(&mdev->md_sync_timer);

    Fixed by moving the del_timer before the test_and_clear_bit.

Additionally only rearm the timer in drbd_md_mark_dirty, if MD_DIRTY was
not already set, reduce the grace period from five to one second, and
add an ifdef'ed debuging aid to find code paths missing an explicit
drbd_md_sync, if any, as those are the only relevant ones for this race.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/block/drbd/drbd_int.h b/drivers/block/drbd/drbd_int.h
index d5e38de83a19..f9b75fc30569 100644
--- a/drivers/block/drbd/drbd_int.h
+++ b/drivers/block/drbd/drbd_int.h
@@ -975,6 +975,12 @@ struct drbd_conf {
                           md_sync_work;
         struct timer_list resync_timer;
         struct timer_list md_sync_timer;
+#ifdef DRBD_DEBUG_MD_SYNC
+        struct {
+                unsigned int line;
+                const char* func;
+        } last_md_mark_dirty;
+#endif
 
         /* Used after attach while negotiating new disk state. */
         union drbd_state new_state_tmp;
@@ -1253,7 +1259,13 @@ extern void drbd_uuid_set_bm(struct drbd_conf *mdev, u64 val) __must_hold(local)
 extern void drbd_md_set_flag(struct drbd_conf *mdev, int flags) __must_hold(local);
 extern void drbd_md_clear_flag(struct drbd_conf *mdev, int flags)__must_hold(local);
 extern int drbd_md_test_flag(struct drbd_backing_dev *, int);
+#ifndef DRBD_DEBUG_MD_SYNC
 extern void drbd_md_mark_dirty(struct drbd_conf *mdev);
+#else
+#define drbd_md_mark_dirty(m)   drbd_md_mark_dirty_(m, __LINE__ , __func__ )
+extern void drbd_md_mark_dirty_(struct drbd_conf *mdev,
+                unsigned int line, const char *func);
+#endif
 extern void drbd_queue_bitmap_io(struct drbd_conf *mdev,
                                  int (*io_fn)(struct drbd_conf *),
                                  void (*done)(struct drbd_conf *, int),