1 /*-------------------------------------------------------------------------
2 *
3 * spin.c
4 * Hardware-independent implementation of spinlocks.
5 *
6 *
7 * For machines that have test-and-set (TAS) instructions, s_lock.h/.c
8 * define the spinlock implementation. This file contains only a stub
9 * implementation for spinlocks using PGSemaphores. Unless semaphores
10 * are implemented in a way that doesn't involve a kernel call, this
11 * is too slow to be very useful :-(
12 *
13 *
14 * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
15 * Portions Copyright (c) 1994, Regents of the University of California
16 *
17 *
18 * IDENTIFICATION
19 * src/backend/storage/lmgr/spin.c
20 *
21 *-------------------------------------------------------------------------
22 */
23 #include "postgres.h"
24
25 #include "storage/pg_sema.h"
26 #include "storage/spin.h"
27
28
29 #ifndef HAVE_SPINLOCKS
30
31 /*
32 * No TAS, so spinlocks are implemented as PGSemaphores.
33 */
34
35 #ifndef HAVE_ATOMICS
36 #define NUM_EMULATION_SEMAPHORES (NUM_SPINLOCK_SEMAPHORES + NUM_ATOMICS_SEMAPHORES)
37 #else
38 #define NUM_EMULATION_SEMAPHORES (NUM_SPINLOCK_SEMAPHORES)
39 #endif /* DISABLE_ATOMICS */
40
41 PGSemaphore SpinlockSemaArray;
42
43 #else /* !HAVE_SPINLOCKS */
44
45 #define NUM_EMULATION_SEMAPHORES 0
46
47 #endif /* HAVE_SPINLOCKS */
48
49 /*
50 * Report the amount of shared memory needed to store semaphores for spinlock
51 * support.
52 */
53 Size
SpinlockSemaSize(void)54 SpinlockSemaSize(void)
55 {
56 return NUM_EMULATION_SEMAPHORES * sizeof(PGSemaphoreData);
57 }
58
59 /*
60 * Report number of semaphores needed to support spinlocks.
61 */
62 int
SpinlockSemas(void)63 SpinlockSemas(void)
64 {
65 return NUM_EMULATION_SEMAPHORES;
66 }
67
68 #ifndef HAVE_SPINLOCKS
69
70 /*
71 * Initialize semaphores.
72 */
73 extern void
SpinlockSemaInit(PGSemaphore spinsemas)74 SpinlockSemaInit(PGSemaphore spinsemas)
75 {
76 int i;
77 int nsemas = SpinlockSemas();
78
79 for (i = 0; i < nsemas; ++i)
80 PGSemaphoreCreate(&spinsemas[i]);
81 SpinlockSemaArray = spinsemas;
82 }
83
84 /*
85 * s_lock.h hardware-spinlock emulation using semaphores
86 *
87 * We map all spinlocks onto NUM_EMULATION_SEMAPHORES semaphores. It's okay to
88 * map multiple spinlocks onto one semaphore because no process should ever
89 * hold more than one at a time. We just need enough semaphores so that we
90 * aren't adding too much extra contention from that.
91 *
92 * There is one exception to the restriction of only holding one spinlock at a
93 * time, which is that it's ok if emulated atomic operations are nested inside
94 * spinlocks. To avoid the danger of spinlocks and atomic using the same sema,
95 * we make sure "normal" spinlocks and atomics backed by spinlocks use
96 * distinct semaphores (see the nested argument to s_init_lock_sema).
97 *
98 * slock_t is just an int for this implementation; it holds the spinlock
99 * number from 1..NUM_EMULATION_SEMAPHORES. We intentionally ensure that 0
100 * is not a valid value, so that testing with this code can help find
101 * failures to initialize spinlocks.
102 */
103
104 static inline void
s_check_valid(int lockndx)105 s_check_valid(int lockndx)
106 {
107 if (lockndx <= 0 || lockndx > NUM_EMULATION_SEMAPHORES)
108 elog(ERROR, "invalid spinlock number: %d", lockndx);
109 }
110
111 void
s_init_lock_sema(volatile slock_t * lock,bool nested)112 s_init_lock_sema(volatile slock_t *lock, bool nested)
113 {
114 static uint32 counter = 0;
115 uint32 offset;
116 uint32 sema_total;
117 uint32 idx;
118
119 if (nested)
120 {
121 /*
122 * To allow nesting atomics inside spinlocked sections, use a
123 * different spinlock. See comment above.
124 */
125 offset = 1 + NUM_SPINLOCK_SEMAPHORES;
126 sema_total = NUM_ATOMICS_SEMAPHORES;
127 }
128 else
129 {
130 offset = 1;
131 sema_total = NUM_SPINLOCK_SEMAPHORES;
132 }
133
134 idx = (counter++ % sema_total) + offset;
135
136 /* double check we did things correctly */
137 s_check_valid(idx);
138
139 *lock = idx;
140 }
141
142 void
s_unlock_sema(volatile slock_t * lock)143 s_unlock_sema(volatile slock_t *lock)
144 {
145 int lockndx = *lock;
146
147 s_check_valid(lockndx);
148
149 PGSemaphoreUnlock(&SpinlockSemaArray[lockndx - 1]);
150 }
151
152 bool
s_lock_free_sema(volatile slock_t * lock)153 s_lock_free_sema(volatile slock_t *lock)
154 {
155 /* We don't currently use S_LOCK_FREE anyway */
156 elog(ERROR, "spin.c does not support S_LOCK_FREE()");
157 return false;
158 }
159
160 int
tas_sema(volatile slock_t * lock)161 tas_sema(volatile slock_t *lock)
162 {
163 int lockndx = *lock;
164
165 s_check_valid(lockndx);
166
167 /* Note that TAS macros return 0 if *success* */
168 return !PGSemaphoreTryLock(&SpinlockSemaArray[lockndx - 1]);
169 }
170
171 #endif /* !HAVE_SPINLOCKS */
172