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         /*
  81          * The TILE architecture doesn't do read speculation; therefore
  82          * a control dependency guarantees a LOAD->{LOAD,STORE} order.
  83          */
  84         barrier();
  85 }
  86 EXPORT_SYMBOL(arch_spin_unlock_wait);
  87
  88 /*
  89  * The low byte is always reserved to be the marker for a "tns" operation
  90  * since the low bit is set to "1" by a tns.  The next seven bits are
  91  * zeroes.  The next byte holds the "next" writer value, i.e. the ticket
  92  * available for the next task that wants to write.  The third byte holds
  93  * the current writer value, i.e. the writer who holds the current ticket.
  94  * If current == next == 0, there are no interested writers.
  95  */
  96 #define WR_NEXT_SHIFT   _WR_NEXT_SHIFT
  97 #define WR_CURR_SHIFT   _WR_CURR_SHIFT
  98 #define WR_WIDTH        _WR_WIDTH
  99 #define WR_MASK         ((1 << WR_WIDTH) - 1)
 100
 101 /*
 102  * The last eight bits hold the active reader count.  This has to be
 103  * zero before a writer can start to write.
 104  */
 105 #define RD_COUNT_SHIFT  _RD_COUNT_SHIFT
 106 #define RD_COUNT_WIDTH  _RD_COUNT_WIDTH
 107 #define RD_COUNT_MASK   ((1 << RD_COUNT_WIDTH) - 1)
 108
 109
 110 /*
 111  * We can get the read lock if everything but the reader bits (which
 112  * are in the high part of the word) is zero, i.e. no active or
 113  * waiting writers, no tns.
 114  *
 115  * We guard the tns/store-back with an interrupt critical section to
 116  * preserve the semantic that the same read lock can be acquired in an
 117  * interrupt context.
 118  */
 119 int arch_read_trylock(arch_rwlock_t *rwlock)
 120 {
 121         u32 val;
 122         __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
 123         val = __insn_tns((int *)&rwlock->lock);
 124         if (likely((val << _RD_COUNT_WIDTH) == 0)) {
 125                 val += 1 << RD_COUNT_SHIFT;
 126                 rwlock->lock = val;
 127                 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
 128                 BUG_ON(val == 0);  /* we don't expect wraparound */
 129                 return 1;
 130         }
 131         if ((val & 1) == 0)
 132                 rwlock->lock = val;
 133         __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
 134         return 0;
 135 }
 136 EXPORT_SYMBOL(arch_read_trylock);
 137
 138 /*
 139  * Spin doing arch_read_trylock() until we acquire the lock.
 140  * ISSUE: This approach can permanently starve readers.  A reader who sees
 141  * a writer could instead take a ticket lock (just like a writer would),
 142  * and atomically enter read mode (with 1 reader) when it gets the ticket.
 143  * This way both readers and writers would always make forward progress
 144  * in a finite time.
 145  */
 146 void arch_read_lock(arch_rwlock_t *rwlock)
 147 {
 148         u32 iterations = 0;
 149         while (unlikely(!arch_read_trylock(rwlock)))
 150                 delay_backoff(iterations++);
 151 }
 152 EXPORT_SYMBOL(arch_read_lock);
 153
 154 void arch_read_unlock(arch_rwlock_t *rwlock)
 155 {
 156         u32 val, iterations = 0;
 157
 158         mb();  /* guarantee anything modified under the lock is visible */
 159         for (;;) {
 160                 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
 161                 val = __insn_tns((int *)&rwlock->lock);
 162                 if (likely((val & 1) == 0)) {
 163                         rwlock->lock = val - (1 << _RD_COUNT_SHIFT);
 164                         __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
 165                         break;
 166                 }
 167                 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
 168                 delay_backoff(iterations++);
 169         }
 170 }
 171 EXPORT_SYMBOL(arch_read_unlock);
 172
 173 /*
 174  * We don't need an interrupt critical section here (unlike for
 175  * arch_read_lock) since we should never use a bare write lock where
 176  * it could be interrupted by code that could try to re-acquire it.
 177  */
 178 void arch_write_lock(arch_rwlock_t *rwlock)
 179 {
 180         /*
 181          * The trailing underscore on this variable (and curr_ below)
 182          * reminds us that the high bits are garbage; we mask them out
 183          * when we compare them.
 184          */
 185         u32 my_ticket_;
 186         u32 iterations = 0;
 187         u32 val = __insn_tns((int *)&rwlock->lock);
 188
 189         if (likely(val == 0)) {
 190                 rwlock->lock = 1 << _WR_NEXT_SHIFT;
 191                 return;
 192         }
 193
 194         /*
 195          * Wait until there are no readers, then bump up the next
 196          * field and capture the ticket value.
 197          */
 198         for (;;) {
 199                 if (!(val & 1)) {
 200                         if ((val >> RD_COUNT_SHIFT) == 0)
 201                                 break;
 202                         rwlock->lock = val;
 203                 }
 204                 delay_backoff(iterations++);
 205                 val = __insn_tns((int *)&rwlock->lock);
 206         }
 207
 208         /* Take out the next ticket and extract my ticket value. */
 209         rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT);
 210         my_ticket_ = val >> WR_NEXT_SHIFT;
 211
 212         /* Wait until the "current" field matches our ticket. */
 213         for (;;) {
 214                 u32 curr_ = val >> WR_CURR_SHIFT;
 215                 u32 delta = ((my_ticket_ - curr_) & WR_MASK);
 216                 if (likely(delta == 0))
 217                         break;
 218
 219                 /* Delay based on how many lock-holders are still out there. */
 220                 relax((256 / CYCLES_PER_RELAX_LOOP) * delta);
 221
 222                 /*
 223                  * Get a non-tns value to check; we don't need to tns
 224                  * it ourselves.  Since we're not tns'ing, we retry
 225                  * more rapidly to get a valid value.
 226                  */
 227                 while ((val = rwlock->lock) & 1)
 228                         relax(4);
 229         }
 230 }
 231 EXPORT_SYMBOL(arch_write_lock);
 232
 233 int arch_write_trylock(arch_rwlock_t *rwlock)
 234 {
 235         u32 val = __insn_tns((int *)&rwlock->lock);
 236
 237         /*
 238          * If a tns is in progress, or there's a waiting or active locker,
 239          * or active readers, we can't take the lock, so give up.
 240          */
 241         if (unlikely(val != 0)) {
 242                 if (!(val & 1))
 243                         rwlock->lock = val;
 244                 return 0;
 245         }
 246
 247         /* Set the "next" field to mark it locked. */
 248         rwlock->lock = 1 << _WR_NEXT_SHIFT;
 249         return 1;
 250 }
 251 EXPORT_SYMBOL(arch_write_trylock);
 252
 253 void arch_write_unlock(arch_rwlock_t *rwlock)
 254 {
 255         u32 val, eq, mask;
 256
 257         mb();  /* guarantee anything modified under the lock is visible */
 258         val = __insn_tns((int *)&rwlock->lock);
 259         if (likely(val == (1 << _WR_NEXT_SHIFT))) {
 260                 rwlock->lock = 0;
 261                 return;
 262         }
 263         while (unlikely(val & 1)) {
 264                 /* Limited backoff since we are the highest-priority task. */
 265                 relax(4);
 266                 val = __insn_tns((int *)&rwlock->lock);
 267         }
 268         mask = 1 << WR_CURR_SHIFT;
 269         val = __insn_addb(val, mask);
 270         eq = __insn_seqb(val, val << (WR_CURR_SHIFT - WR_NEXT_SHIFT));
 271         val = __insn_mz(eq & mask, val);
 272         rwlock->lock = val;
 273 }
 274 EXPORT_SYMBOL(arch_write_unlock);