arch/tile/lib/spinlock_32.c

   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
  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
  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
  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  */
 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  */
 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
 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  */
 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
 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
 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);