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 /*
81 * The TILE architecture doesn't do read speculation; therefore
82 * a control dependency guarantees a LOAD->{LOAD,STORE} order.
83 */
84 barrier();
85 }
86 EXPORT_SYMBOL(arch_spin_unlock_wait);
87
88 /*
89 * The low byte is always reserved to be the marker for a "tns" operation
90 * since the low bit is set to "1" by a tns. The next seven bits are
91 * zeroes. The next byte holds the "next" writer value, i.e. the ticket
92 * available for the next task that wants to write. The third byte holds
93 * the current writer value, i.e. the writer who holds the current ticket.
94 * If current == next == 0, there are no interested writers.
95 */
96 #define WR_NEXT_SHIFT _WR_NEXT_SHIFT
97 #define WR_CURR_SHIFT _WR_CURR_SHIFT
98 #define WR_WIDTH _WR_WIDTH
99 #define WR_MASK ((1 << WR_WIDTH) - 1)
100
101 /*
102 * The last eight bits hold the active reader count. This has to be
103 * zero before a writer can start to write.
104 */
105 #define RD_COUNT_SHIFT _RD_COUNT_SHIFT
106 #define RD_COUNT_WIDTH _RD_COUNT_WIDTH
107 #define RD_COUNT_MASK ((1 << RD_COUNT_WIDTH) - 1)
108
109
110 /*
111 * We can get the read lock if everything but the reader bits (which
112 * are in the high part of the word) is zero, i.e. no active or
113 * waiting writers, no tns.
114 *
115 * We guard the tns/store-back with an interrupt critical section to
116 * preserve the semantic that the same read lock can be acquired in an
117 * interrupt context.
118 */
arch_read_trylock(arch_rwlock_t * rwlock)119 int arch_read_trylock(arch_rwlock_t *rwlock)
120 {
121 u32 val;
122 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
123 val = __insn_tns((int *)&rwlock->lock);
124 if (likely((val << _RD_COUNT_WIDTH) == 0)) {
125 val += 1 << RD_COUNT_SHIFT;
126 rwlock->lock = val;
127 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
128 BUG_ON(val == 0); /* we don't expect wraparound */
129 return 1;
130 }
131 if ((val & 1) == 0)
132 rwlock->lock = val;
133 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
134 return 0;
135 }
136 EXPORT_SYMBOL(arch_read_trylock);
137
138 /*
139 * Spin doing arch_read_trylock() until we acquire the lock.
140 * ISSUE: This approach can permanently starve readers. A reader who sees
141 * a writer could instead take a ticket lock (just like a writer would),
142 * and atomically enter read mode (with 1 reader) when it gets the ticket.
143 * This way both readers and writers would always make forward progress
144 * in a finite time.
145 */
arch_read_lock(arch_rwlock_t * rwlock)146 void arch_read_lock(arch_rwlock_t *rwlock)
147 {
148 u32 iterations = 0;
149 while (unlikely(!arch_read_trylock(rwlock)))
150 delay_backoff(iterations++);
151 }
152 EXPORT_SYMBOL(arch_read_lock);
153
arch_read_unlock(arch_rwlock_t * rwlock)154 void arch_read_unlock(arch_rwlock_t *rwlock)
155 {
156 u32 val, iterations = 0;
157
158 mb(); /* guarantee anything modified under the lock is visible */
159 for (;;) {
160 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
161 val = __insn_tns((int *)&rwlock->lock);
162 if (likely((val & 1) == 0)) {
163 rwlock->lock = val - (1 << _RD_COUNT_SHIFT);
164 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
165 break;
166 }
167 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
168 delay_backoff(iterations++);
169 }
170 }
171 EXPORT_SYMBOL(arch_read_unlock);
172
173 /*
174 * We don't need an interrupt critical section here (unlike for
175 * arch_read_lock) since we should never use a bare write lock where
176 * it could be interrupted by code that could try to re-acquire it.
177 */
arch_write_lock(arch_rwlock_t * rwlock)178 void arch_write_lock(arch_rwlock_t *rwlock)
179 {
180 /*
181 * The trailing underscore on this variable (and curr_ below)
182 * reminds us that the high bits are garbage; we mask them out
183 * when we compare them.
184 */
185 u32 my_ticket_;
186 u32 iterations = 0;
187 u32 val = __insn_tns((int *)&rwlock->lock);
188
189 if (likely(val == 0)) {
190 rwlock->lock = 1 << _WR_NEXT_SHIFT;
191 return;
192 }
193
194 /*
195 * Wait until there are no readers, then bump up the next
196 * field and capture the ticket value.
197 */
198 for (;;) {
199 if (!(val & 1)) {
200 if ((val >> RD_COUNT_SHIFT) == 0)
201 break;
202 rwlock->lock = val;
203 }
204 delay_backoff(iterations++);
205 val = __insn_tns((int *)&rwlock->lock);
206 }
207
208 /* Take out the next ticket and extract my ticket value. */
209 rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT);
210 my_ticket_ = val >> WR_NEXT_SHIFT;
211
212 /* Wait until the "current" field matches our ticket. */
213 for (;;) {
214 u32 curr_ = val >> WR_CURR_SHIFT;
215 u32 delta = ((my_ticket_ - curr_) & WR_MASK);
216 if (likely(delta == 0))
217 break;
218
219 /* Delay based on how many lock-holders are still out there. */
220 relax((256 / CYCLES_PER_RELAX_LOOP) * delta);
221
222 /*
223 * Get a non-tns value to check; we don't need to tns
224 * it ourselves. Since we're not tns'ing, we retry
225 * more rapidly to get a valid value.
226 */
227 while ((val = rwlock->lock) & 1)
228 relax(4);
229 }
230 }
231 EXPORT_SYMBOL(arch_write_lock);
232
arch_write_trylock(arch_rwlock_t * rwlock)233 int arch_write_trylock(arch_rwlock_t *rwlock)
234 {
235 u32 val = __insn_tns((int *)&rwlock->lock);
236
237 /*
238 * If a tns is in progress, or there's a waiting or active locker,
239 * or active readers, we can't take the lock, so give up.
240 */
241 if (unlikely(val != 0)) {
242 if (!(val & 1))
243 rwlock->lock = val;
244 return 0;
245 }
246
247 /* Set the "next" field to mark it locked. */
248 rwlock->lock = 1 << _WR_NEXT_SHIFT;
249 return 1;
250 }
251 EXPORT_SYMBOL(arch_write_trylock);
252
arch_write_unlock(arch_rwlock_t * rwlock)253 void arch_write_unlock(arch_rwlock_t *rwlock)
254 {
255 u32 val, eq, mask;
256
257 mb(); /* guarantee anything modified under the lock is visible */
258 val = __insn_tns((int *)&rwlock->lock);
259 if (likely(val == (1 << _WR_NEXT_SHIFT))) {
260 rwlock->lock = 0;
261 return;
262 }
263 while (unlikely(val & 1)) {
264 /* Limited backoff since we are the highest-priority task. */
265 relax(4);
266 val = __insn_tns((int *)&rwlock->lock);
267 }
268 mask = 1 << WR_CURR_SHIFT;
269 val = __insn_addb(val, mask);
270 eq = __insn_seqb(val, val << (WR_CURR_SHIFT - WR_NEXT_SHIFT));
271 val = __insn_mz(eq & mask, val);
272 rwlock->lock = val;
273 }
274 EXPORT_SYMBOL(arch_write_unlock);
275