Commit 3efa66ce authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Peter Zijlstra
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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 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: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250416162921.513656-2-bigeasy@linutronix.de
parent b4432656

include/linux/rcuref.h

+21 −1
Original line number Diff line number Diff line
@@ -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);



/**