- Enforce an implicit lock order that Giant cannot be locked while holding

  any other non-sleepable lock.  In plain English: Giant comes before all
  other mutexes.
- Add some extra description to the lock order reversal printf's to indicate
  when a reversal is triggered by a hard-coded implicit rule.

Requested by:	truckman (2)
MFC after:	1 week

1 files changed, 17 insertions, 1 deletions

diff --git a/sys/kern/subr_witness.c b/sys/kern/subr_witness.c
index e94b017..7f038f9 100644
--- a/sys/kern/subr_witness.c
+++ b/sys/kern/subr_witness.c
@@ -960,6 +960,13 @@ witness_checkorder(struct lock_object *lock, int flags, const char *file,
 			    (lock1->li_lock->lo_flags & LO_SLEEPABLE) == 0))
 				goto reversal;
 			/*
+			 * If we are locking Giant and this is a non-sleepable
+			 * lock, then treat it as a reversal.
+			 */
+			if ((lock1->li_lock->lo_flags & LO_SLEEPABLE) == 0 &&
+			    lock == &Giant.mtx_object)
+				goto reversal;
+			/*
 			 * Check the lock order hierarchy for a reveresal.
 			 */
 			if (!isitmydescendant(w, w1))
@@ -993,7 +1000,16 @@ witness_checkorder(struct lock_object *lock, int flags, const char *file,
 			/*
 			 * Ok, yell about it.
 			 */
-			printf("lock order reversal\n");
+			if (((lock->lo_flags & LO_SLEEPABLE) != 0 &&
+			    (lock1->li_lock->lo_flags & LO_SLEEPABLE) == 0))
+				printf(
+		"lock order reversal: (sleepable after non-sleepable)\n");
+			else if ((lock1->li_lock->lo_flags & LO_SLEEPABLE) == 0
+			    && lock == &Giant.mtx_object)
+				printf(
+		"lock order reversal: (Giant after non-sleepable)\n");
+			else
+				printf("lock order reversal:\n");
 			/*
 			 * Try to locate an earlier lock with
 			 * witness w in our list.