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         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  */
 104 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  */
 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
 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  */
 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
 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
 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);