Hazard pointers

Safe reclamation for lock-free data structures: readers publish "I'm using @c p" hazard records so writers know not to free p until every reader has moved on. More...

Files
file	rhzrptr.h
	Hazard pointers — safe reclamation for lock-free reads.

Data Structures
struct	rhzrptr
	Pointer slot that participates in hazard-pointer reclamation. More...

Macros
#define	R_HZR_PTR_INIT(notify)   { 0, (RDestroyNotify)notify }
	Static initialiser for an rhzrptr with destroy notifier notify.

Typedefs
typedef struct RHzrPtrRec	RHzrPtrRec
	Opaque per-thread hazard record.

Functions
rpointer	r_hzr_ptr_acquire (rhzrptr *hzrptr, RHzrPtrRec *rec)
	Publish a hazard for the current value of hzrptr and return it.
void	r_hzr_ptr_release (rhzrptr *hzrptr, RHzrPtrRec *rec)
	Drop the hazard published by a previous r_hzr_ptr_acquire.
void	r_hzr_ptr_replace (rhzrptr *hzrptr, rpointer ptr)
	Atomically replace hzrptr's value with ptr and schedule the displaced value for delayed reclamation.
RHzrPtrRec *	r_hzr_ptr_rec_new (void)
	Allocate a per-thread hazard record.
void	r_hzr_ptr_rec_free (RHzrPtrRec *rec)
	Free a per-thread hazard record at thread exit.

Detailed Description

Safe reclamation for lock-free data structures: readers publish "I'm using @c p" hazard records so writers know not to free p until every reader has moved on.

The shape mirrors the canonical Michael 2004 design:

• Each reader thread owns an RHzrPtrRec (created via r_hzr_ptr_rec_new, freed at thread exit with r_hzr_ptr_rec_free).
• Accesses to a hazard pointer happen between r_hzr_ptr_acquire and r_hzr_ptr_release.
• Writers swap pointers via r_hzr_ptr_replace, which defers freeing the displaced value until no RHzrPtrRec still references it. The per-pointer destroy notifier (stored on the rhzrptr) runs at that point.

Function Documentation

r_hzr_ptr_acquire()

rpointer r_hzr_ptr_acquire	(	rhzrptr *	hzrptr,
		RHzrPtrRec *	rec
	)

Publish a hazard for the current value of hzrptr and return it.

Pair with r_hzr_ptr_release once the caller is done with the returned pointer; while a hazard is active on the value, writers will defer freeing it.

r_hzr_ptr_replace()

void r_hzr_ptr_replace	(	rhzrptr *	hzrptr,
		rpointer	ptr
	)

Atomically replace hzrptr's value with ptr and schedule the displaced value for delayed reclamation.

The previous value is freed via hzrptr->notify once no RHzrPtrRec still holds a hazard on it.