gfs2: do_promote glock holder stealing fix

In do_promote(), when the glock had no strong holders, we were
accidentally calling demote_incompat_holders() with new_gh == NULL, so
no weak holders were considered incompatible.  Instead, the new holder
should have been passed in.

For doing that, the HIF_HOLDER flag needs to be set in new_gh to prevent
may_grant() from complaining.  This means that the new holder will now
be recognized as a current holder, so skip over it explicitly in
demote_incompat_holders() to prevent it from being dequeued.

To further clarify things, we can now rename new_gh to current_gh in
demote_incompat_holders(); after all, the HIF_HOLDER flag is already set,
which means the new holder is already a current holder.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/glock.c

@@ -405,10 +405,13 @@ static void do_error(struct gfs2_glock *gl, const int ret)
 /**
  * demote_incompat_holders - demote incompatible demoteable holders
  * @gl: the glock we want to promote
- * @new_gh: the new holder to be promoted
+ * @current_gh: the newly promoted holder
+ *
+ * We're passing the newly promoted holder in @current_gh, but actually, any of
+ * the strong holders would do.
  */
 static void demote_incompat_holders(struct gfs2_glock *gl,
-				    struct gfs2_holder *new_gh)
+				    struct gfs2_holder *current_gh)
 {
 	struct gfs2_holder *gh, *tmp;
 
@@ -424,8 +427,10 @@ static void demote_incompat_holders(struct gfs2_glock *gl,
 		 */
 		if (!test_bit(HIF_HOLDER, &gh->gh_iflags))
 			return;
+		if (gh == current_gh)
+			continue;
 		if (test_bit(HIF_MAY_DEMOTE, &gh->gh_iflags) &&
-		    !may_grant(gl, new_gh, gh)) {
+		    !may_grant(gl, current_gh, gh)) {
 			/*
 			 * We should not recurse into do_promote because
 			 * __gfs2_glock_dq only calls handle_callback,
@@ -547,14 +552,14 @@ static int do_promote(struct gfs2_glock *gl)
 			do_error(gl, 0);
 			break;
 		}
+		set_bit(HIF_HOLDER, &gh->gh_iflags);
+		trace_gfs2_promote(gh);
+		gfs2_holder_wake(gh);
 		if (!incompat_holders_demoted) {
+			current_gh = gh;
 			demote_incompat_holders(gl, current_gh);
 			incompat_holders_demoted = true;
-			current_gh = gh;
 		}
-		set_bit(HIF_HOLDER, &gh->gh_iflags);
-		trace_gfs2_promote(gh);
-		gfs2_holder_wake(gh);
 	}
 	return 0;
 }