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