1 /*
2 * Copyright 2010 Tilera Corporation. All Rights Reserved.
3 *
4 * This program is free software; you can redistribute it and/or
5 * modify it under the terms of the GNU General Public License
6 * as published by the Free Software Foundation, version 2.
7 *
8 * This program is distributed in the hope that it will be useful, but
9 * WITHOUT ANY WARRANTY; without even the implied warranty of
10 * MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
11 * NON INFRINGEMENT. See the GNU General Public License for
12 * more details.
13 */
14
15 #include <linux/spinlock.h>
16 #include <linux/module.h>
17 #include <asm/processor.h>
18 #include <arch/spr_def.h>
19
20 #include "spinlock_common.h"
21
arch_spin_lock(arch_spinlock_t * lock)22 void arch_spin_lock(arch_spinlock_t *lock)
23 {
24 int my_ticket;
25 int iterations = 0;
26 int delta;
27
28 while ((my_ticket = __insn_tns((void *)&lock->next_ticket)) & 1)
29 delay_backoff(iterations++);
30
31 /* Increment the next ticket number, implicitly releasing tns lock. */
32 lock->next_ticket = my_ticket + TICKET_QUANTUM;
33
34 /* Wait until it's our turn. */
35 while ((delta = my_ticket - lock->current_ticket) != 0)
36 relax((128 / CYCLES_PER_RELAX_LOOP) * delta);
37 }
38 EXPORT_SYMBOL(arch_spin_lock);
39
arch_spin_trylock(arch_spinlock_t * lock)40 int arch_spin_trylock(arch_spinlock_t *lock)
41 {
42 /*
43 * Grab a ticket; no need to retry if it's busy, we'll just
44 * treat that the same as "locked", since someone else
45 * will lock it momentarily anyway.
46 */
47 int my_ticket = __insn_tns((void *)&lock->next_ticket);
48
49 if (my_ticket == lock->current_ticket) {
50 /* Not currently locked, so lock it by keeping this ticket. */
51 lock->next_ticket = my_ticket + TICKET_QUANTUM;
52 /* Success! */
53 return 1;
54 }
55
56 if (!(my_ticket & 1)) {
57 /* Release next_ticket. */
58 lock->next_ticket = my_ticket;
59 }
60
61 return 0;
62 }
63 EXPORT_SYMBOL(arch_spin_trylock);
64
65 /*
66 * The low byte is always reserved to be the marker for a "tns" operation
67 * since the low bit is set to "1" by a tns. The next seven bits are
68 * zeroes. The next byte holds the "next" writer value, i.e. the ticket
69 * available for the next task that wants to write. The third byte holds
70 * the current writer value, i.e. the writer who holds the current ticket.
71 * If current == next == 0, there are no interested writers.
72 */
73 #define WR_NEXT_SHIFT _WR_NEXT_SHIFT
74 #define WR_CURR_SHIFT _WR_CURR_SHIFT
75 #define WR_WIDTH _WR_WIDTH
76 #define WR_MASK ((1 << WR_WIDTH) - 1)
77
78 /*
79 * The last eight bits hold the active reader count. This has to be
80 * zero before a writer can start to write.
81 */
82 #define RD_COUNT_SHIFT _RD_COUNT_SHIFT
83 #define RD_COUNT_WIDTH _RD_COUNT_WIDTH
84 #define RD_COUNT_MASK ((1 << RD_COUNT_WIDTH) - 1)
85
86
87 /*
88 * We can get the read lock if everything but the reader bits (which
89 * are in the high part of the word) is zero, i.e. no active or
90 * waiting writers, no tns.
91 *
92 * We guard the tns/store-back with an interrupt critical section to
93 * preserve the semantic that the same read lock can be acquired in an
94 * interrupt context.
95 */
arch_read_trylock(arch_rwlock_t * rwlock)96 int arch_read_trylock(arch_rwlock_t *rwlock)
97 {
98 u32 val;
99 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
100 val = __insn_tns((int *)&rwlock->lock);
101 if (likely((val << _RD_COUNT_WIDTH) == 0)) {
102 val += 1 << RD_COUNT_SHIFT;
103 rwlock->lock = val;
104 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
105 BUG_ON(val == 0); /* we don't expect wraparound */
106 return 1;
107 }
108 if ((val & 1) == 0)
109 rwlock->lock = val;
110 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
111 return 0;
112 }
113 EXPORT_SYMBOL(arch_read_trylock);
114
115 /*
116 * Spin doing arch_read_trylock() until we acquire the lock.
117 * ISSUE: This approach can permanently starve readers. A reader who sees
118 * a writer could instead take a ticket lock (just like a writer would),
119 * and atomically enter read mode (with 1 reader) when it gets the ticket.
120 * This way both readers and writers would always make forward progress
121 * in a finite time.
122 */
arch_read_lock(arch_rwlock_t * rwlock)123 void arch_read_lock(arch_rwlock_t *rwlock)
124 {
125 u32 iterations = 0;
126 while (unlikely(!arch_read_trylock(rwlock)))
127 delay_backoff(iterations++);
128 }
129 EXPORT_SYMBOL(arch_read_lock);
130
arch_read_unlock(arch_rwlock_t * rwlock)131 void arch_read_unlock(arch_rwlock_t *rwlock)
132 {
133 u32 val, iterations = 0;
134
135 mb(); /* guarantee anything modified under the lock is visible */
136 for (;;) {
137 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
138 val = __insn_tns((int *)&rwlock->lock);
139 if (likely((val & 1) == 0)) {
140 rwlock->lock = val - (1 << _RD_COUNT_SHIFT);
141 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
142 break;
143 }
144 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
145 delay_backoff(iterations++);
146 }
147 }
148 EXPORT_SYMBOL(arch_read_unlock);
149
150 /*
151 * We don't need an interrupt critical section here (unlike for
152 * arch_read_lock) since we should never use a bare write lock where
153 * it could be interrupted by code that could try to re-acquire it.
154 */
arch_write_lock(arch_rwlock_t * rwlock)155 void arch_write_lock(arch_rwlock_t *rwlock)
156 {
157 /*
158 * The trailing underscore on this variable (and curr_ below)
159 * reminds us that the high bits are garbage; we mask them out
160 * when we compare them.
161 */
162 u32 my_ticket_;
163 u32 iterations = 0;
164 u32 val = __insn_tns((int *)&rwlock->lock);
165
166 if (likely(val == 0)) {
167 rwlock->lock = 1 << _WR_NEXT_SHIFT;
168 return;
169 }
170
171 /*
172 * Wait until there are no readers, then bump up the next
173 * field and capture the ticket value.
174 */
175 for (;;) {
176 if (!(val & 1)) {
177 if ((val >> RD_COUNT_SHIFT) == 0)
178 break;
179 rwlock->lock = val;
180 }
181 delay_backoff(iterations++);
182 val = __insn_tns((int *)&rwlock->lock);
183 }
184
185 /* Take out the next ticket and extract my ticket value. */
186 rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT);
187 my_ticket_ = val >> WR_NEXT_SHIFT;
188
189 /* Wait until the "current" field matches our ticket. */
190 for (;;) {
191 u32 curr_ = val >> WR_CURR_SHIFT;
192 u32 delta = ((my_ticket_ - curr_) & WR_MASK);
193 if (likely(delta == 0))
194 break;
195
196 /* Delay based on how many lock-holders are still out there. */
197 relax((256 / CYCLES_PER_RELAX_LOOP) * delta);
198
199 /*
200 * Get a non-tns value to check; we don't need to tns
201 * it ourselves. Since we're not tns'ing, we retry
202 * more rapidly to get a valid value.
203 */
204 while ((val = rwlock->lock) & 1)
205 relax(4);
206 }
207 }
208 EXPORT_SYMBOL(arch_write_lock);
209
arch_write_trylock(arch_rwlock_t * rwlock)210 int arch_write_trylock(arch_rwlock_t *rwlock)
211 {
212 u32 val = __insn_tns((int *)&rwlock->lock);
213
214 /*
215 * If a tns is in progress, or there's a waiting or active locker,
216 * or active readers, we can't take the lock, so give up.
217 */
218 if (unlikely(val != 0)) {
219 if (!(val & 1))
220 rwlock->lock = val;
221 return 0;
222 }
223
224 /* Set the "next" field to mark it locked. */
225 rwlock->lock = 1 << _WR_NEXT_SHIFT;
226 return 1;
227 }
228 EXPORT_SYMBOL(arch_write_trylock);
229
arch_write_unlock(arch_rwlock_t * rwlock)230 void arch_write_unlock(arch_rwlock_t *rwlock)
231 {
232 u32 val, eq, mask;
233
234 mb(); /* guarantee anything modified under the lock is visible */
235 val = __insn_tns((int *)&rwlock->lock);
236 if (likely(val == (1 << _WR_NEXT_SHIFT))) {
237 rwlock->lock = 0;
238 return;
239 }
240 while (unlikely(val & 1)) {
241 /* Limited backoff since we are the highest-priority task. */
242 relax(4);
243 val = __insn_tns((int *)&rwlock->lock);
244 }
245 mask = 1 << WR_CURR_SHIFT;
246 val = __insn_addb(val, mask);
247 eq = __insn_seqb(val, val << (WR_CURR_SHIFT - WR_NEXT_SHIFT));
248 val = __insn_mz(eq & mask, val);
249 rwlock->lock = val;
250 }
251 EXPORT_SYMBOL(arch_write_unlock);
252