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
41 struct ck_pflock {
42 uint32_t rin;
43 uint32_t rout;
44 uint32_t win;
45 uint32_t wout;
46 };
47 typedef 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
57 CK_CC_INLINE static void
ck_pflock_init(struct ck_pflock * pf)58 ck_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
70 CK_CC_INLINE static void
ck_pflock_write_unlock(ck_pflock_t * pf)71 ck_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
84 CK_CC_INLINE static void
ck_pflock_write_lock(ck_pflock_t * pf)85 ck_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
110 CK_CC_INLINE static void
ck_pflock_read_unlock(ck_pflock_t * pf)111 ck_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
119 CK_CC_INLINE static void
ck_pflock_read_lock(ck_pflock_t * pf)120 ck_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
136 leave:
137 /* Acquire semantics with respect to readers. */
138 ck_pr_fence_lock();
139 return;
140 }
141
142 #endif /* CK_PFLOCK_H */
143