Re: lockdep pinning

[Peter Zijlstra]

On Thu, Feb 26, 2026 at 05:01:08PM -0500, Liam R. Howlett wrote:
> Hello,
> 
> I'm trying to detect locking issues in the maple tree, which is severely
> complicated by allowing external locks.  I have an rcu lock validation
> using the get_state_synchronize_rcu() and poll_state_synchronize_rcu()
> interface working and would like something like this for write locks.
> 
> The issue is that, with external locks, the code that does the unlocking
> is outside my scope.  Although I can detect a start of an operation and
> pin a lock using the existing lockdep_pin_lock() function, I do not know
> when the lock is 'done' as it can be used for multiple operations on the
> tree.
> 
> So I'd either be unpinning too early and be unable to tell if it was
> unlocked/relocked, or I'd leave it pinned forever - which will trigger a
> warning on release.
> 
> What I'm looking for is a way to ensure my code is still in the same
> lock context, but not unpin/pin.  Maybe something like storing a lock
> cookie id in the lock every time it's locked (just overwrite would be
> fine), and a way to read that value.  Then I can validate the value
> against the one I've stored.
> 
> AFAICT, I cannot accomplish what I want with the existing lockdep
> implementation?

Right, that seems to be the exact opposite of that the pin thing was
build for. That was to ensure nobody else releases my lock, whereas you
want to make sure you're in the same critical section of this other
person's lock.

So if you have LOCK_STAT=y, you could use hlock->holdtime_stamp for this
I suppose, but that is a bit yuck.

How's something like so? It isn't really nice, but should mostly work I
recon.

When consecutive lockdep_sequence(your_lock) calls return a different
value you can assume it ain't the same critical section no moar.

---

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 621566345406..a6451ecbbe9a 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -273,6 +273,9 @@ extern struct pin_cookie lock_pin_lock(struct lockdep_map *lock);
 extern void lock_repin_lock(struct lockdep_map *lock, struct pin_cookie);
 extern void lock_unpin_lock(struct lockdep_map *lock, struct pin_cookie);
 
+extern u32 lock_sequence(struct lockdep_map *lock);
+#define lockdep_sequence(lock)	lock_sequence(&(lock)->dep_map)
+
 #define lockdep_depth(tsk)	(debug_locks ? (tsk)->lockdep_depth : 0)
 
 #define lockdep_assert(cond)		\
diff --git a/include/linux/lockdep_types.h b/include/linux/lockdep_types.h
index eae115a26488..55c4b152fedf 100644
--- a/include/linux/lockdep_types.h
+++ b/include/linux/lockdep_types.h
@@ -253,7 +253,8 @@ struct held_lock {
 	unsigned int hardirqs_off:1;
 	unsigned int sync:1;
 	unsigned int references:11;					/* 32 bits */
-	unsigned int pin_count;
+	unsigned int pin_count:24;
+	unsigned int seq_count:8;
 };
 
 #else /* !CONFIG_LOCKDEP */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 074ad4ef3d81..41a440685c32 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1267,6 +1267,7 @@ struct task_struct {
 	u64				curr_chain_key;
 	int				lockdep_depth;
 	unsigned int			lockdep_recursion;
+	unsigned int			lockdep_seq;
 	struct held_lock		held_locks[MAX_LOCK_DEPTH];
 #endif
 
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 2d4c5bab5af8..b2b0a506960b 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -5077,7 +5077,7 @@ static int __lock_is_held(const struct lockdep_map *lock, int read);
 static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 			  int trylock, int read, int check, int hardirqs_off,
 			  struct lockdep_map *nest_lock, unsigned long ip,
-			  int references, int pin_count, int sync)
+			  int references, int pin_count, int sync, int seq)
 {
 	struct task_struct *curr = current;
 	struct lock_class *class = NULL;
@@ -5183,6 +5183,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	hlock->holdtime_stamp = lockstat_clock();
 #endif
 	hlock->pin_count = pin_count;
+	hlock->seq_count = seq;
 
 	if (check_wait_context(curr, hlock))
 		return 0;
@@ -5388,7 +5389,7 @@ static int reacquire_held_locks(struct task_struct *curr, unsigned int depth,
 				    hlock->read, hlock->check,
 				    hlock->hardirqs_off,
 				    hlock->nest_lock, hlock->acquire_ip,
-				    hlock->references, hlock->pin_count, 0)) {
+				    hlock->references, hlock->pin_count, 0, hlock->seq_count)) {
 		case 0:
 			return 1;
 		case 1:
@@ -5684,6 +5685,24 @@ static void __lock_unpin_lock(struct lockdep_map *lock, struct pin_cookie cookie
 	WARN(1, "unpinning an unheld lock\n");
 }
 
+static u32 __lock_sequence(struct lockdep_map *lock)
+{
+	struct task_struct *curr = current;
+	int i;
+
+	if (unlikely(!debug_locks))
+		return ~0;
+
+	for (i = 0; i < curr->lockdep_depth; i++) {
+		struct held_lock *hlock = curr->held_locks + i;
+
+		if (match_held_lock(hlock, lock))
+			return hlock->seq_count;
+	}
+
+	return ~0;
+}
+
 /*
  * Check whether we follow the irq-flags state precisely:
  */
@@ -5866,7 +5885,8 @@ void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 
 	lockdep_recursion_inc();
 	__lock_acquire(lock, subclass, trylock, read, check,
-		       irqs_disabled_flags(flags), nest_lock, ip, 0, 0, 0);
+		       irqs_disabled_flags(flags), nest_lock, ip, 0, 0, 0,
+		       ++current->lockdep_seq);
 	lockdep_recursion_finish();
 	raw_local_irq_restore(flags);
 }
@@ -5914,7 +5934,8 @@ void lock_sync(struct lockdep_map *lock, unsigned subclass, int read,
 
 	lockdep_recursion_inc();
 	__lock_acquire(lock, subclass, 0, read, check,
-		       irqs_disabled_flags(flags), nest_lock, ip, 0, 0, 1);
+		       irqs_disabled_flags(flags), nest_lock, ip, 0, 0, 1,
+		       ++current->lockdep_seq);
 	check_chain_key(current);
 	lockdep_recursion_finish();
 	raw_local_irq_restore(flags);
@@ -6000,6 +6021,26 @@ void lock_unpin_lock(struct lockdep_map *lock, struct pin_cookie cookie)
 }
 EXPORT_SYMBOL_GPL(lock_unpin_lock);
 
+u32 lock_sequence(struct lockdep_map *lock)
+{
+	unsigned long flags;
+	u32 seq = ~0;
+
+	if (unlikely(!lockdep_enabled()))
+		return seq;
+
+	raw_local_irq_save(flags);
+	check_flags(flags);
+
+	lockdep_recursion_inc();
+	seq = __lock_sequence(lock);
+	lockdep_recursion_finish();
+	raw_local_irq_restore(flags);
+
+	return seq;
+}
+EXPORT_SYMBOL_GPL(lock_sequence);
+
 #ifdef CONFIG_LOCK_STAT
 static void print_lock_contention_bug(struct task_struct *curr,
 				      struct lockdep_map *lock,