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
arch_spin_unlock_wait(arch_spinlock_t * lock)65 void arch_spin_unlock_wait(arch_spinlock_t *lock)
66 {
67 u32 iterations = 0;
68 int curr = READ_ONCE(lock->current_ticket);
69 int next = READ_ONCE(lock->next_ticket);
70
71 /* Return immediately if unlocked. */
72 if (next == curr)
73 return;
74
75 /* Wait until the current locker has released the lock. */
76 do {
77 delay_backoff(iterations++);
78 } while (READ_ONCE(lock->current_ticket) == curr);
79 }
80 EXPORT_SYMBOL(arch_spin_unlock_wait);
81
82 /*
83 * The low byte is always reserved to be the marker for a "tns" operation
84 * since the low bit is set to "1" by a tns. The next seven bits are
85 * zeroes. The next byte holds the "next" writer value, i.e. the ticket
86 * available for the next task that wants to write. The third byte holds
87 * the current writer value, i.e. the writer who holds the current ticket.
88 * If current == next == 0, there are no interested writers.
89 */
90 #define WR_NEXT_SHIFT _WR_NEXT_SHIFT
91 #define WR_CURR_SHIFT _WR_CURR_SHIFT
92 #define WR_WIDTH _WR_WIDTH
93 #define WR_MASK ((1 << WR_WIDTH) - 1)
94
95 /*
96 * The last eight bits hold the active reader count. This has to be
97 * zero before a writer can start to write.
98 */
99 #define RD_COUNT_SHIFT _RD_COUNT_SHIFT
100 #define RD_COUNT_WIDTH _RD_COUNT_WIDTH
101 #define RD_COUNT_MASK ((1 << RD_COUNT_WIDTH) - 1)
102
103
104 /*
105 * We can get the read lock if everything but the reader bits (which
106 * are in the high part of the word) is zero, i.e. no active or
107 * waiting writers, no tns.
108 *
109 * We guard the tns/store-back with an interrupt critical section to
110 * preserve the semantic that the same read lock can be acquired in an
111 * interrupt context.
112 */
arch_read_trylock(arch_rwlock_t * rwlock)113 int arch_read_trylock(arch_rwlock_t *rwlock)
114 {
115 u32 val;
116 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
117 val = __insn_tns((int *)&rwlock->lock);
118 if (likely((val << _RD_COUNT_WIDTH) == 0)) {
119 val += 1 << RD_COUNT_SHIFT;
120 rwlock->lock = val;
121 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
122 BUG_ON(val == 0); /* we don't expect wraparound */
123 return 1;
124 }
125 if ((val & 1) == 0)
126 rwlock->lock = val;
127 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
128 return 0;
129 }
130 EXPORT_SYMBOL(arch_read_trylock);
131
132 /*
133 * Spin doing arch_read_trylock() until we acquire the lock.
134 * ISSUE: This approach can permanently starve readers. A reader who sees
135 * a writer could instead take a ticket lock (just like a writer would),
136 * and atomically enter read mode (with 1 reader) when it gets the ticket.
137 * This way both readers and writers would always make forward progress
138 * in a finite time.
139 */
arch_read_lock(arch_rwlock_t * rwlock)140 void arch_read_lock(arch_rwlock_t *rwlock)
141 {
142 u32 iterations = 0;
143 while (unlikely(!arch_read_trylock(rwlock)))
144 delay_backoff(iterations++);
145 }
146 EXPORT_SYMBOL(arch_read_lock);
147
arch_read_unlock(arch_rwlock_t * rwlock)148 void arch_read_unlock(arch_rwlock_t *rwlock)
149 {
150 u32 val, iterations = 0;
151
152 mb(); /* guarantee anything modified under the lock is visible */
153 for (;;) {
154 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
155 val = __insn_tns((int *)&rwlock->lock);
156 if (likely((val & 1) == 0)) {
157 rwlock->lock = val - (1 << _RD_COUNT_SHIFT);
158 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
159 break;
160 }
161 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
162 delay_backoff(iterations++);
163 }
164 }
165 EXPORT_SYMBOL(arch_read_unlock);
166
167 /*
168 * We don't need an interrupt critical section here (unlike for
169 * arch_read_lock) since we should never use a bare write lock where
170 * it could be interrupted by code that could try to re-acquire it.
171 */
arch_write_lock(arch_rwlock_t * rwlock)172 void arch_write_lock(arch_rwlock_t *rwlock)
173 {
174 /*
175 * The trailing underscore on this variable (and curr_ below)
176 * reminds us that the high bits are garbage; we mask them out
177 * when we compare them.
178 */
179 u32 my_ticket_;
180 u32 iterations = 0;
181 u32 val = __insn_tns((int *)&rwlock->lock);
182
183 if (likely(val == 0)) {
184 rwlock->lock = 1 << _WR_NEXT_SHIFT;
185 return;
186 }
187
188 /*
189 * Wait until there are no readers, then bump up the next
190 * field and capture the ticket value.
191 */
192 for (;;) {
193 if (!(val & 1)) {
194 if ((val >> RD_COUNT_SHIFT) == 0)
195 break;
196 rwlock->lock = val;
197 }
198 delay_backoff(iterations++);
199 val = __insn_tns((int *)&rwlock->lock);
200 }
201
202 /* Take out the next ticket and extract my ticket value. */
203 rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT);
204 my_ticket_ = val >> WR_NEXT_SHIFT;
205
206 /* Wait until the "current" field matches our ticket. */
207 for (;;) {
208 u32 curr_ = val >> WR_CURR_SHIFT;
209 u32 delta = ((my_ticket_ - curr_) & WR_MASK);
210 if (likely(delta == 0))
211 break;
212
213 /* Delay based on how many lock-holders are still out there. */
214 relax((256 / CYCLES_PER_RELAX_LOOP) * delta);
215
216 /*
217 * Get a non-tns value to check; we don't need to tns
218 * it ourselves. Since we're not tns'ing, we retry
219 * more rapidly to get a valid value.
220 */
221 while ((val = rwlock->lock) & 1)
222 relax(4);
223 }
224 }
225 EXPORT_SYMBOL(arch_write_lock);
226
arch_write_trylock(arch_rwlock_t * rwlock)227 int arch_write_trylock(arch_rwlock_t *rwlock)
228 {
229 u32 val = __insn_tns((int *)&rwlock->lock);
230
231 /*
232 * If a tns is in progress, or there's a waiting or active locker,
233 * or active readers, we can't take the lock, so give up.
234 */
235 if (unlikely(val != 0)) {
236 if (!(val & 1))
237 rwlock->lock = val;
238 return 0;
239 }
240
241 /* Set the "next" field to mark it locked. */
242 rwlock->lock = 1 << _WR_NEXT_SHIFT;
243 return 1;
244 }
245 EXPORT_SYMBOL(arch_write_trylock);
246
arch_write_unlock(arch_rwlock_t * rwlock)247 void arch_write_unlock(arch_rwlock_t *rwlock)
248 {
249 u32 val, eq, mask;
250
251 mb(); /* guarantee anything modified under the lock is visible */
252 val = __insn_tns((int *)&rwlock->lock);
253 if (likely(val == (1 << _WR_NEXT_SHIFT))) {
254 rwlock->lock = 0;
255 return;
256 }
257 while (unlikely(val & 1)) {
258 /* Limited backoff since we are the highest-priority task. */
259 relax(4);
260 val = __insn_tns((int *)&rwlock->lock);
261 }
262 mask = 1 << WR_CURR_SHIFT;
263 val = __insn_addb(val, mask);
264 eq = __insn_seqb(val, val << (WR_CURR_SHIFT - WR_NEXT_SHIFT));
265 val = __insn_mz(eq & mask, val);
266 rwlock->lock = val;
267 }
268 EXPORT_SYMBOL(arch_write_unlock);
269