Re: [PATCH v9 00/11] futex: Add support task local hash maps.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2025-03-11 11:17:14 [+0100], Peter Zijlstra wrote:
> Right, so I failed to understand initially. When DEAD it stays 0, but
> there is indeed the one case where it isn't yet DEAD but still returns
> 0.
> 
> Making the DEAD return -1 seems like a good solution.

The patch below is what I have/ tglx asked for. I intend to use it the
series and repost it once I fixed it up.

-------------->8--------------

Subject: [PATCH] rcuref: Provide rcuref_is_dead().

rcuref_read() returns the number of references that are currently held.
If 0 is returned then it is not safe to assume that the object ca be
scheduled for deconstruction because it is marked DEAD. This happens if
the return value of rcuref_put() is ignored and assumptions are made.

If 0 is returned then the counter transitioned from 0 to RCUREF_NOREF.
If rcuref_put() did not return to the caller then the counter did not
yet transition from RCUREF_NOREF to RCUREF_DEAD. This means that there
is still a chance that the counter counter will transition from
RCUREF_NOREF to 0 meaning it is still valid and must not be
deconstructed. In this brief window rcuref_read() will return 0.

Provide rcuref_is_dead() to determine if the counter is marked as
RCUREF_DEAD.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/rcuref.h | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/include/linux/rcuref.h b/include/linux/rcuref.h
index 6322d8c1c6b42..2fb2af6d98249 100644
--- a/include/linux/rcuref.h
+++ b/include/linux/rcuref.h
@@ -30,7 +30,11 @@ static inline void rcuref_init(rcuref_t *ref, unsigned int cnt)
  * rcuref_read - Read the number of held reference counts of a rcuref
  * @ref:	Pointer to the reference count
  *
- * Return: The number of held references (0 ... N)
+ * Return: The number of held references (0 ... N). The value 0 does not
+ * indicate that it is safe to schedule the object, protected by this reference
+ * counter, for deconstruction.
+ * If you want to know if the reference counter has been marked DEAD (as
+ * signaled by rcuref_put()) please use rcuread_is_dead().
  */
 static inline unsigned int rcuref_read(rcuref_t *ref)
 {
@@ -40,6 +44,22 @@ static inline unsigned int rcuref_read(rcuref_t *ref)
 	return c >= RCUREF_RELEASED ? 0 : c + 1;
 }
 
+/**
+ * rcuref_is_dead -	Check if the rcuref has been already marked dead
+ * @ref:		Pointer to the reference count
+ *
+ * Return: True if the object has been marked DEAD. This signals that a previous
+ * invocation of rcuref_put() returned true on this reference counter meaning
+ * the protected object can safely be scheduled for deconstruction.
+ * Otherwise, returns false.
+ */
+static inline bool rcuref_is_dead(rcuref_t *ref)
+{
+	unsigned int c = atomic_read(&ref->refcnt);
+
+	return (c >= RCUREF_RELEASED) && (c < RCUREF_NOREF);
+}
+
 extern __must_check bool rcuref_get_slowpath(rcuref_t *ref);
 
 /**
-- 
2.47.2