aboutsummaryrefslogtreecommitdiff
path: root/arch/tile/lib/spinlock_32.c
blob: b34f79aada48efca7a1b350894f0dd7f4b98a7f4 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
/*
 * Copyright 2010 Tilera Corporation. All Rights Reserved.
 *
 *   This program is free software; you can redistribute it and/or
 *   modify it under the terms of the GNU General Public License
 *   as published by the Free Software Foundation, version 2.
 *
 *   This program is distributed in the hope that it will be useful, but
 *   WITHOUT ANY WARRANTY; without even the implied warranty of
 *   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
 *   NON INFRINGEMENT.  See the GNU General Public License for
 *   more details.
 */

#include <linux/spinlock.h>
#include <linux/module.h>
#include <asm/processor.h>
#include <arch/spr_def.h>

#include "spinlock_common.h"

void arch_spin_lock(arch_spinlock_t *lock)
{
	int my_ticket;
	int iterations = 0;
	int delta;

	while ((my_ticket = __insn_tns((void *)&lock->next_ticket)) & 1)
		delay_backoff(iterations++);

	/* Increment the next ticket number, implicitly releasing tns lock. */
	lock->next_ticket = my_ticket + TICKET_QUANTUM;

	/* Wait until it's our turn. */
	while ((delta = my_ticket - lock->current_ticket) != 0)
		relax((128 / CYCLES_PER_RELAX_LOOP) * delta);
}
EXPORT_SYMBOL(arch_spin_lock);

int arch_spin_trylock(arch_spinlock_t *lock)
{
	/*
	 * Grab a ticket; no need to retry if it's busy, we'll just
	 * treat that the same as "locked", since someone else
	 * will lock it momentarily anyway.
	 */
	int my_ticket = __insn_tns((void *)&lock->next_ticket);

	if (my_ticket == lock->current_ticket) {
		/* Not currently locked, so lock it by keeping this ticket. */
		lock->next_ticket = my_ticket + TICKET_QUANTUM;
		/* Success! */
		return 1;
	}

	if (!(my_ticket & 1)) {
		/* Release next_ticket. */
		lock->next_ticket = my_ticket;
	}

	return 0;
}
EXPORT_SYMBOL(arch_spin_trylock);

void arch_spin_unlock_wait(arch_spinlock_t *lock)
{
	u32 iterations = 0;
	while (arch_spin_is_locked(lock))
		delay_backoff(iterations++);
}
EXPORT_SYMBOL(arch_spin_unlock_wait);

/*
 * The low byte is always reserved to be the marker for a "tns" operation
 * since the low bit is set to "1" by a tns.  The next seven bits are
 * zeroes.  The next byte holds the "next" writer value, i.e. the ticket
 * available for the next task that wants to write.  The third byte holds
 * the current writer value, i.e. the writer who holds the current ticket.
 * If current == next == 0, there are no interested writers.
 */
#define WR_NEXT_SHIFT   _WR_NEXT_SHIFT
#define WR_CURR_SHIFT   _WR_CURR_SHIFT
#define WR_WIDTH        _WR_WIDTH
#define WR_MASK         ((1 << WR_WIDTH) - 1)

/*
 * The last eight bits hold the active reader count.  This has to be
 * zero before a writer can start to write.
 */
#define RD_COUNT_SHIFT  _RD_COUNT_SHIFT
#define RD_COUNT_WIDTH  _RD_COUNT_WIDTH
#define RD_COUNT_MASK   ((1 << RD_COUNT_WIDTH) - 1)


/*
 * We can get the read lock if everything but the reader bits (which
 * are in the high part of the word) is zero, i.e. no active or
 * waiting writers, no tns.
 *
 * We guard the tns/store-back with an interrupt critical section to
 * preserve the semantic that the same read lock can be acquired in an
 * interrupt context.
 */
int arch_read_trylock(arch_rwlock_t *rwlock)
{
	u32 val;
	__insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
	val = __insn_tns((int *)&rwlock->lock);
	if (likely((val << _RD_COUNT_WIDTH) == 0)) {
		val += 1 << RD_COUNT_SHIFT;
		rwlock->lock = val;
		__insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
		BUG_ON(val == 0);  /* we don't expect wraparound */
		return 1;
	}
	if ((val & 1) == 0)
		rwlock->lock = val;
	__insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
	return 0;
}
EXPORT_SYMBOL(arch_read_trylock);

/*
 * Spin doing arch_read_trylock() until we acquire the lock.
 * ISSUE: This approach can permanently starve readers.  A reader who sees
 * a writer could instead take a ticket lock (just like a writer would),
 * and atomically enter read mode (with 1 reader) when it gets the ticket.
 * This way both readers and writers would always make forward progress
 * in a finite time.
 */
void arch_read_lock(arch_rwlock_t *rwlock)
{
	u32 iterations = 0;
	while (unlikely(!arch_read_trylock(rwlock)))
		delay_backoff(iterations++);
}
EXPORT_SYMBOL(arch_read_lock);

void arch_read_unlock(arch_rwlock_t *rwlock)
{
	u32 val, iterations = 0;

	mb();  /* guarantee anything modified under the lock is visible */
	for (;;) {
		__insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
		val = __insn_tns((int *)&rwlock->lock);
		if (likely((val & 1) == 0)) {
			rwlock->lock = val - (1 << _RD_COUNT_SHIFT);
			__insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
			break;
		}
		__insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
		delay_backoff(iterations++);
	}
}
EXPORT_SYMBOL(arch_read_unlock);

/*
 * We don't need an interrupt critical section here (unlike for
 * arch_read_lock) since we should never use a bare write lock where
 * it could be interrupted by code that could try to re-acquire it.
 */
void arch_write_lock(arch_rwlock_t *rwlock)
{
	/*
	 * The trailing underscore on this variable (and curr_ below)
	 * reminds us that the high bits are garbage; we mask them out
	 * when we compare them.
	 */
	u32 my_ticket_;
	u32 iterations = 0;
	u32 val = __insn_tns((int *)&rwlock->lock);

	if (likely(val == 0)) {
		rwlock->lock = 1 << _WR_NEXT_SHIFT;
		return;
	}

	/*
	 * Wait until there are no readers, then bump up the next
	 * field and capture the ticket value.
	 */
	for (;;) {
		if (!(val & 1)) {
			if ((val >> RD_COUNT_SHIFT) == 0)
				break;
			rwlock->lock = val;
		}
		delay_backoff(iterations++);
		val = __insn_tns((int *)&rwlock->lock);
	}

	/* Take out the next ticket and extract my ticket value. */
	rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT);
	my_ticket_ = val >> WR_NEXT_SHIFT;

	/* Wait until the "current" field matches our ticket. */
	for (;;) {
		u32 curr_ = val >> WR_CURR_SHIFT;
		u32 delta = ((my_ticket_ - curr_) & WR_MASK);
		if (likely(delta == 0))
			break;

		/* Delay based on how many lock-holders are still out there. */
		relax((256 / CYCLES_PER_RELAX_LOOP) * delta);

		/*
		 * Get a non-tns value to check; we don't need to tns
		 * it ourselves.  Since we're not tns'ing, we retry
		 * more rapidly to get a valid value.
		 */
		while ((val = rwlock->lock) & 1)
			relax(4);
	}
}
EXPORT_SYMBOL(arch_write_lock);

int arch_write_trylock(arch_rwlock_t *rwlock)
{
	u32 val = __insn_tns((int *)&rwlock->lock);

	/*
	 * If a tns is in progress, or there's a waiting or active locker,
	 * or active readers, we can't take the lock, so give up.
	 */
	if (unlikely(val != 0)) {
		if (!(val & 1))
			rwlock->lock = val;
		return 0;
	}

	/* Set the "next" field to mark it locked. */
	rwlock->lock = 1 << _WR_NEXT_SHIFT;
	return 1;
}
EXPORT_SYMBOL(arch_write_trylock);

void arch_write_unlock(arch_rwlock_t *rwlock)
{
	u32 val, eq, mask;

	mb();  /* guarantee anything modified under the lock is visible */
	val = __insn_tns((int *)&rwlock->lock);
	if (likely(val == (1 << _WR_NEXT_SHIFT))) {
		rwlock->lock = 0;
		return;
	}
	while (unlikely(val & 1)) {
		/* Limited backoff since we are the highest-priority task. */
		relax(4);
		val = __insn_tns((int *)&rwlock->lock);
	}
	mask = 1 << WR_CURR_SHIFT;
	val = __insn_addb(val, mask);
	eq = __insn_seqb(val, val << (WR_CURR_SHIFT - WR_NEXT_SHIFT));
	val = __insn_mz(eq & mask, val);
	rwlock->lock = val;
}
EXPORT_SYMBOL(arch_write_unlock);