linux/arch/tile/lib/spinlock_32.c
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   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
  22void 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}
  38EXPORT_SYMBOL(arch_spin_lock);
  39
  40int 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}
  63EXPORT_SYMBOL(arch_spin_trylock);
  64
  65void 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}
  71EXPORT_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 */
 104int 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}
 121EXPORT_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 */
 131void arch_read_lock(arch_rwlock_t *rwlock)
 132{
 133        u32 iterations = 0;
 134        while (unlikely(!arch_read_trylock(rwlock)))
 135                delay_backoff(iterations++);
 136}
 137EXPORT_SYMBOL(arch_read_lock);
 138
 139void 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}
 156EXPORT_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 */
 163void 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}
 216EXPORT_SYMBOL(arch_write_lock);
 217
 218int 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}
 236EXPORT_SYMBOL(arch_write_trylock);
 237
 238void 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}
 259EXPORT_SYMBOL(arch_write_unlock);
 260