ck/include/ck_epoch.h

/*
 * Copyright 2011-2015 Samy Al Bahra.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#ifndef CK_EPOCH_H
#define CK_EPOCH_H

/*
 * The implementation here is inspired from the work described in:
 *   Fraser, K. 2004. Practical Lock-Freedom. PhD Thesis, University
 *   of Cambridge Computing Laboratory.
 */

#include <ck_cc.h>
#include <ck_md.h>
#include <ck_pr.h>
#include <ck_stack.h>
#include <ck_stdbool.h>

#ifndef CK_EPOCH_LENGTH
#define CK_EPOCH_LENGTH 4
#endif

/*
 * This is used for sense detection with-respect to concurrent
 * epoch sections.
 */
#define CK_EPOCH_SENSE		(2)

struct ck_epoch_entry;
typedef struct ck_epoch_entry ck_epoch_entry_t;
typedef void ck_epoch_cb_t(ck_epoch_entry_t *);

/*
 * This should be embedded into objects you wish to be the target of
 * ck_epoch_cb_t functions (with ck_epoch_call).
 */
struct ck_epoch_entry {
	ck_epoch_cb_t *function;
	ck_stack_entry_t stack_entry;
};

/*
 * A section object may be passed to every begin-end pair to allow for
 * forward progress guarantees with-in prolonged active sections.
 */
struct ck_epoch_section {
	unsigned int bucket;
};
typedef struct ck_epoch_section ck_epoch_section_t;

/*
 * Return pointer to ck_epoch_entry container object.
 */
#define CK_EPOCH_CONTAINER(T, M, N) \
	CK_CC_CONTAINER(struct ck_epoch_entry, T, M, N)

struct ck_epoch_ref {
	unsigned int epoch;
	unsigned int count;
};

struct ck_epoch_record {
	struct ck_epoch *global;
	unsigned int state;
	unsigned int epoch;
	unsigned int active;
	struct {
		struct ck_epoch_ref bucket[CK_EPOCH_SENSE];
	} local CK_CC_CACHELINE;
	unsigned int n_pending;
	unsigned int n_peak;
	unsigned int n_dispatch;
	void *ct;
	ck_stack_t pending[CK_EPOCH_LENGTH];
	ck_stack_entry_t record_next;
} CK_CC_CACHELINE;
typedef struct ck_epoch_record ck_epoch_record_t;

struct ck_epoch {
	unsigned int epoch;
	char pad[CK_MD_CACHELINE - sizeof(unsigned int)];
	ck_stack_t records;
	unsigned int n_free;
};
typedef struct ck_epoch ck_epoch_t;

/*
 * Internal functions.
 */
void _ck_epoch_addref(ck_epoch_record_t *, ck_epoch_section_t *);
void _ck_epoch_delref(ck_epoch_record_t *, ck_epoch_section_t *);

CK_CC_FORCE_INLINE static void *
ck_epoch_record_ct(const ck_epoch_record_t *record)
{

	return ck_pr_load_ptr(&record->ct);
}

/*
 * Marks the beginning of an epoch-protected section.
 */
CK_CC_FORCE_INLINE static void
ck_epoch_begin(ck_epoch_record_t *record, ck_epoch_section_t *section)
{
	struct ck_epoch *epoch = record->global;

	/*
	 * Only observe new epoch if thread is not recursing into a read
	 * section.
	 */
	if (record->active == 0) {
		unsigned int g_epoch;

		/*
		 * It is possible for loads to be re-ordered before the store
		 * is committed into the caller's epoch and active fields.
		 * For this reason, store to load serialization is necessary.
		 */
#if defined(CK_MD_TSO)
		ck_pr_fas_uint(&record->active, 1);
		ck_pr_fence_atomic_load();
#else
		ck_pr_store_uint(&record->active, 1);
		ck_pr_fence_memory();
#endif

		/*
		 * This load is allowed to be re-ordered prior to setting
		 * active flag due to monotonic nature of the global epoch.
		 * However, stale values lead to measurable performance
		 * degradation in some torture tests so we disallow early load
		 * of global epoch.
		 */
		g_epoch = ck_pr_load_uint(&epoch->epoch);
		ck_pr_store_uint(&record->epoch, g_epoch);
	} else {
		ck_pr_store_uint(&record->active, record->active + 1);
	}

	if (section != NULL)
		_ck_epoch_addref(record, section);

	return;
}

/*
 * Marks the end of an epoch-protected section. Returns true if no more
 * sections exist for the caller.
 */
CK_CC_FORCE_INLINE static bool
ck_epoch_end(ck_epoch_record_t *record, ck_epoch_section_t *section)
{

	ck_pr_fence_release();
	ck_pr_store_uint(&record->active, record->active - 1);

	if (section != NULL)
		return _ck_epoch_delref(record, section);

	return record->active == 0;
}

/*
 * Defers the execution of the function pointed to by the "cb"
 * argument until an epoch counter loop. This allows for a
 * non-blocking deferral.
 *
 * We can get away without a fence here due to the monotonic nature
 * of the epoch counter. Worst case, this will result in some delays
 * before object destruction.
 */
CK_CC_FORCE_INLINE static void
ck_epoch_call(ck_epoch_record_t *record,
	      ck_epoch_entry_t *entry,
	      ck_epoch_cb_t *function)
{
	struct ck_epoch *epoch = record->global;
	unsigned int e = ck_pr_load_uint(&epoch->epoch);
	unsigned int offset = e & (CK_EPOCH_LENGTH - 1);

	record->n_pending++;
	entry->function = function;
	ck_stack_push_spnc(&record->pending[offset], &entry->stack_entry);
	return;
}

/*
 * Same as ck_epoch_call, but allows for records to be shared and is reentrant.
 */
CK_CC_FORCE_INLINE static void
ck_epoch_call_strict(ck_epoch_record_t *record,
	      ck_epoch_entry_t *entry,
	      ck_epoch_cb_t *function)
{
	struct ck_epoch *epoch = record->global;
	unsigned int e = ck_pr_load_uint(&epoch->epoch);
	unsigned int offset = e & (CK_EPOCH_LENGTH - 1);

	ck_pr_inc_uint(&record->n_pending);
	entry->function = function;

	/* Store fence is implied by push operation. */
	ck_stack_push_upmc(&record->pending[offset], &entry->stack_entry);
	return;
}

/*
 * This callback is used for synchronize_wait to allow for custom blocking
 * behavior.
 */
typedef void ck_epoch_wait_cb_t(ck_epoch_t *, ck_epoch_record_t *,
    void *);

/*
 * Return latest epoch value. This operation provides load ordering.
 */
CK_CC_FORCE_INLINE static unsigned int
ck_epoch_value(const ck_epoch_t *ep)
{

	ck_pr_fence_load();
	return ck_pr_load_uint(&ep->epoch);
}

void ck_epoch_init(ck_epoch_t *);

/*
 * Attempts to recycle an unused epoch record. If one is successfully
 * allocated, the record context pointer is also updated.
 */
ck_epoch_record_t *ck_epoch_recycle(ck_epoch_t *, void *);

/*
 * Registers an epoch record. An optional context pointer may be passed that
 * is retrievable with ck_epoch_record_ct.
 */
void ck_epoch_register(ck_epoch_t *, ck_epoch_record_t *, void *);

/*
 * Marks a record as available for re-use by a subsequent recycle operation.
 * Note that the record cannot be physically destroyed.
 */
void ck_epoch_unregister(ck_epoch_record_t *);

bool ck_epoch_poll(ck_epoch_record_t *);
void ck_epoch_synchronize(ck_epoch_record_t *);
void ck_epoch_synchronize_wait(ck_epoch_t *, ck_epoch_wait_cb_t *, void *);
void ck_epoch_barrier(ck_epoch_record_t *);
void ck_epoch_barrier_wait(ck_epoch_record_t *, ck_epoch_wait_cb_t *, void *);

/*
 * Reclaim entries associated with a record. This is safe to call only on
 * the caller's record or records that are using call_strict.
 */
void ck_epoch_reclaim(ck_epoch_record_t *);

#endif /* CK_EPOCH_H */