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rcuref: Provide rcuref_is_dead()

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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: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250416162921.513656-2-bigeasy@linutronix.de
This commit is contained in:
Sebastian Andrzej Siewior 2025-04-16 18:29:01 +02:00 committed by Peter Zijlstra
parent b4432656b3
commit 3efa66ce6e
1 changed files with 21 additions and 1 deletions
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22
include/linux/rcuref.h
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@ -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);
/**