1/*
2 * Copyright 2013 John Wittrock.
3 * Copyright 2013-2015 Samy Al Bahra.
4 * All rights reserved.
5 *
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
8 * are met:
9 * 1. Redistributions of source code must retain the above copyright
10 *    notice, this list of conditions and the following disclaimer.
11 * 2. Redistributions in binary form must reproduce the above copyright
12 *    notice, this list of conditions and the following disclaimer in the
13 *    documentation and/or other materials provided with the distribution.
14 *
15 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
16 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
17 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
18 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
19 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
20 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
21 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
22 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
23 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
24 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
25 * SUCH DAMAGE.
26 */
27
28#ifndef CK_PFLOCK_H
29#define CK_PFLOCK_H
30
31/*
32 * This is an implementation of phase-fair locks derived from the work
33 * described in:
34 *    Brandenburg, B. and Anderson, J. 2010. Spin-Based
35 *    Reader-Writer Synchronization for Multiprocessor Real-Time Systems
36 */
37
38#include <ck_cc.h>
39#include <ck_pr.h>
40
41struct ck_pflock {
42	uint32_t rin;
43	uint32_t rout;
44	uint32_t win;
45	uint32_t wout;
46};
47typedef struct ck_pflock ck_pflock_t;
48
49#define CK_PFLOCK_LSB   0xFFFFFFF0
50#define CK_PFLOCK_RINC  0x100		/* Reader increment value. */
51#define CK_PFLOCK_WBITS 0x3		/* Writer bits in reader. */
52#define CK_PFLOCK_PRES  0x2		/* Writer present bit. */
53#define CK_PFLOCK_PHID  0x1		/* Phase ID bit. */
54
55#define CK_PFLOCK_INITIALIZER {0, 0, 0, 0}
56
57CK_CC_INLINE static void
58ck_pflock_init(struct ck_pflock *pf)
59{
60
61	pf->rin = 0;
62	pf->rout = 0;
63	pf->win = 0;
64	pf->wout = 0;
65	ck_pr_barrier();
66
67	return;
68}
69
70CK_CC_INLINE static void
71ck_pflock_write_unlock(ck_pflock_t *pf)
72{
73
74	ck_pr_fence_unlock();
75
76	/* Migrate from write phase to read phase. */
77	ck_pr_and_32(&pf->rin, CK_PFLOCK_LSB);
78
79	/* Allow other writers to continue. */
80	ck_pr_faa_32(&pf->wout, 1);
81	return;
82}
83
84CK_CC_INLINE static void
85ck_pflock_write_lock(ck_pflock_t *pf)
86{
87	uint32_t ticket;
88
89	/* Acquire ownership of write-phase. */
90	ticket = ck_pr_faa_32(&pf->win, 1);
91	while (ck_pr_load_32(&pf->wout) != ticket)
92		ck_pr_stall();
93
94	/*
95	 * Acquire ticket on read-side in order to allow them
96	 * to flush. Indicates to any incoming reader that a
97	 * write-phase is pending.
98	 */
99	ticket = ck_pr_faa_32(&pf->rin,
100	    (ticket & CK_PFLOCK_PHID) | CK_PFLOCK_PRES);
101
102	/* Wait for any pending readers to flush. */
103	while (ck_pr_load_32(&pf->rout) != ticket)
104		ck_pr_stall();
105
106	ck_pr_fence_lock();
107	return;
108}
109
110CK_CC_INLINE static void
111ck_pflock_read_unlock(ck_pflock_t *pf)
112{
113
114	ck_pr_fence_unlock();
115	ck_pr_faa_32(&pf->rout, CK_PFLOCK_RINC);
116	return;
117}
118
119CK_CC_INLINE static void
120ck_pflock_read_lock(ck_pflock_t *pf)
121{
122	uint32_t w;
123
124	/*
125	 * If no writer is present, then the operation has completed
126	 * successfully.
127	 */
128	w = ck_pr_faa_32(&pf->rin, CK_PFLOCK_RINC) & CK_PFLOCK_WBITS;
129	if (w == 0)
130		goto leave;
131
132	/* Wait for current write phase to complete. */
133	while ((ck_pr_load_32(&pf->rin) & CK_PFLOCK_WBITS) == w)
134		ck_pr_stall();
135
136leave:
137	/* Acquire semantics with respect to readers. */
138	ck_pr_fence_lock();
139	return;
140}
141
142#endif /* CK_PFLOCK_H */
143