x15/kern/sref.h

/*
 * Copyright (c) 2014-2019 Richard Braun.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 *
 * Scalable reference counting.
 *
 * The purpose of this module is to reduce the amount of inter-processor
 * communication usually involved with reference counting. Scalable
 * reference counters should only be used when multiprocessor scalability
 * is important because of the costs they imply (increased memory usage
 * and latencies).
 *
 * When a counter drops to 0, the no-reference function associated with it
 * is called in work context. As a result, special care must be taken if
 * using sref counters in the work module itself.
 */

#ifndef KERN_SREF_H
#define KERN_SREF_H

#include <stdint.h>

#include <kern/init.h>
#include <kern/slist.h>
#include <kern/spinlock.h>
#include <kern/work.h>

struct sref_counter;

// Type for no-reference functions.
typedef void (*sref_noref_fn_t) (struct sref_counter *);

#ifdef CONFIG_SREF_DEBUG
  #define SREF_VERIFY
#endif

/*
 * Weak reference.
 *
 * A weak reference is a pointer to a reference counter in which the
 * least-significant bit is used to indicate whether the counter is
 * "dying", i.e. about to be destroyed.
 *
 * It must be accessed with atomic instructions. There is no need to
 * enforce memory order on access since the only data that depends on
 * the weak reference are cpu-local deltas.
 */
struct sref_weakref
{
  uintptr_t addr;
};

/*
 * Scalable reference counter.
 *
 * It's tempting to merge the flags into the node member, but since they're
 * not protected by the same lock, store them separately.
 *
 * Locking keys :
 * (c) sref_counter
 * (g) sref_data
 *
 * Interrupts must be disabled when accessing a global counter.
 */
struct sref_counter
{
  sref_noref_fn_t noref_fn;

#ifdef SREF_VERIFY
  struct
#else
  union
#endif
    {
      struct
        {
          struct slist_node node;         // (g)
          struct spinlock lock;
          int flags;                      // (c)
          unsigned long value;            // (c)
          struct sref_weakref *weakref;
        };

      struct work work;
    };
};

/*
 * Report a periodic event (normally the periodic timer interrupt) on the
 * current processor.
 *
 * Interrupts and preemption must be disabled when calling this function.
 */
void sref_report_periodic_event (void);

/*
 * Initialize a scalable reference counter.
 *
 * The no-reference function is called (from thread context) when it is
 * certain that the true number of references is 0.
 */
void sref_counter_init (struct sref_counter *counter,
                        unsigned long init_value,
                        struct sref_weakref *weakref,
                        sref_noref_fn_t noref_fn);

/*
 * Counter operations.
 *
 * These functions may safely be called in interrupt context.
 *
 * These functions imply a compiler barrier.
 */
void sref_counter_inc (struct sref_counter *counter);
void sref_counter_dec (struct sref_counter *counter);

/*
 * Attempt to get a reference from a weak reference.
 *
 * If successful, increment the reference counter before returning it.
 * Otherwise return NULL.
 *
 * This function may safely be called in interrupt context.
 */
struct sref_counter* sref_weakref_get (struct sref_weakref *weakref);

/*
 * This init operation provides :
 *  - sref counter and weakref initialization and usage
 */
INIT_OP_DECLARE (sref_bootstrap);

#endif