Re: [PATCH] locking/lockdep: Fix expected depth value in __lock_release()

[Peter Zijlstra]

On Mon, Oct 19, 2015 at 03:30:28PM -0400, j.glisse@xxxxxxxxx wrote:
> From: Jérôme Glisse <jglisse@xxxxxxxxxx>
> 
> In __lock_release() we are removing one entry from the stack and
> rebuilding the hash chain by re-adding entry above the entry we
> just removed. If the entry removed was between 2 entry of same
> class then this 2 entry might be coalesced into one single entry
> which in turns means that the lockdep_depth value will not be
> incremented and thus the expected lockdep_depth value after this
> operation will be wrong triggering an unjustified WARN_ONCE() at
> the end of __lock_release().

This is the nest_lock stuff, right? Where it checks:

  if (hlock->class_idx == class_idx && nest_lock) {
	...
	return 1;
  }

What code did you find that triggered this? That is, what code is taking
nested locks with other locks in the middle? (Not wrong per-se, just
curious how that would come about).

> This patch adjust the expect depth value by decrementing it if
> what was previously 2 entry inside the stack are coalesced into
> only one entry.

Would it not make more sense to scan the entire hlock stack on
__lock_acquire() and avoid getting collapsible entries in the first
place?

Something like so...

---
 kernel/locking/lockdep.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 4e49cc4c9952..6fcd98b86e7b 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3125,15 +3125,21 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 
 	class_idx = class - lock_classes + 1;
 
-	if (depth) {
-		hlock = curr->held_locks + depth - 1;
-		if (hlock->class_idx == class_idx && nest_lock) {
-			if (hlock->references)
-				hlock->references++;
-			else
-				hlock->references = 2;
+	if (depth && nest_lock) {
+		int i;
 
-			return 1;
+		for (i = depth; i; --i) {
+			hlock = curr->held_locks + i - 1;
+			if (hlock->class_idx == class_idx &&
+			    hlock->nest_lock == nest_lock) {
+
+				if (hlock->references)
+					hlock->references++;
+				else
+					hlock->references = 2;
+
+				return 1;
+			}
 		}
 	}
 
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