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 /*
  66  * The low byte is always reserved to be the marker for a "tns" operation
  67  * since the low bit is set to "1" by a tns.  The next seven bits are
  68  * zeroes.  The next byte holds the "next" writer value, i.e. the ticket
  69  * available for the next task that wants to write.  The third byte holds
  70  * the current writer value, i.e. the writer who holds the current ticket.
  71  * If current == next == 0, there are no interested writers.
  72  */
  73 #define WR_NEXT_SHIFT   _WR_NEXT_SHIFT
  74 #define WR_CURR_SHIFT   _WR_CURR_SHIFT
  75 #define WR_WIDTH        _WR_WIDTH
  76 #define WR_MASK         ((1 << WR_WIDTH) - 1)
  77
  78 /*
  79  * The last eight bits hold the active reader count.  This has to be
  80  * zero before a writer can start to write.
  81  */
  82 #define RD_COUNT_SHIFT  _RD_COUNT_SHIFT
  83 #define RD_COUNT_WIDTH  _RD_COUNT_WIDTH
  84 #define RD_COUNT_MASK   ((1 << RD_COUNT_WIDTH) - 1)
  85
  86
  87 /*
  88  * We can get the read lock if everything but the reader bits (which
  89  * are in the high part of the word) is zero, i.e. no active or
  90  * waiting writers, no tns.
  91  *
  92  * We guard the tns/store-back with an interrupt critical section to
  93  * preserve the semantic that the same read lock can be acquired in an
  94  * interrupt context.
  95  */
  96 int arch_read_trylock(arch_rwlock_t *rwlock)
  97 {
  98         u32 val;
  99         __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
 100         val = __insn_tns((int *)&rwlock->lock);
 101         if (likely((val << _RD_COUNT_WIDTH) == 0)) {
 102                 val += 1 << RD_COUNT_SHIFT;
 103                 rwlock->lock = val;
 104                 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
 105                 BUG_ON(val == 0);  /* we don't expect wraparound */
 106                 return 1;
 107         }
 108         if ((val & 1) == 0)
 109                 rwlock->lock = val;
 110         __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
 111         return 0;
 112 }
 113 EXPORT_SYMBOL(arch_read_trylock);
 114
 115 /*
 116  * Spin doing arch_read_trylock() until we acquire the lock.
 117  * ISSUE: This approach can permanently starve readers.  A reader who sees
 118  * a writer could instead take a ticket lock (just like a writer would),
 119  * and atomically enter read mode (with 1 reader) when it gets the ticket.
 120  * This way both readers and writers would always make forward progress
 121  * in a finite time.
 122  */
 123 void arch_read_lock(arch_rwlock_t *rwlock)
 124 {
 125         u32 iterations = 0;
 126         while (unlikely(!arch_read_trylock(rwlock)))
 127                 delay_backoff(iterations++);
 128 }
 129 EXPORT_SYMBOL(arch_read_lock);
 130
 131 void arch_read_unlock(arch_rwlock_t *rwlock)
 132 {
 133         u32 val, iterations = 0;
 134
 135         mb();  /* guarantee anything modified under the lock is visible */
 136         for (;;) {
 137                 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);
 138                 val = __insn_tns((int *)&rwlock->lock);
 139                 if (likely((val & 1) == 0)) {
 140                         rwlock->lock = val - (1 << _RD_COUNT_SHIFT);
 141                         __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
 142                         break;
 143                 }
 144                 __insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
 145                 delay_backoff(iterations++);
 146         }
 147 }
 148 EXPORT_SYMBOL(arch_read_unlock);
 149
 150 /*
 151  * We don't need an interrupt critical section here (unlike for
 152  * arch_read_lock) since we should never use a bare write lock where
 153  * it could be interrupted by code that could try to re-acquire it.
 154  */
 155 void arch_write_lock(arch_rwlock_t *rwlock)
 156 {
 157         /*
 158          * The trailing underscore on this variable (and curr_ below)
 159          * reminds us that the high bits are garbage; we mask them out
 160          * when we compare them.
 161          */
 162         u32 my_ticket_;
 163         u32 iterations = 0;
 164         u32 val = __insn_tns((int *)&rwlock->lock);
 165
 166         if (likely(val == 0)) {
 167                 rwlock->lock = 1 << _WR_NEXT_SHIFT;
 168                 return;
 169         }
 170
 171         /*
 172          * Wait until there are no readers, then bump up the next
 173          * field and capture the ticket value.
 174          */
 175         for (;;) {
 176                 if (!(val & 1)) {
 177                         if ((val >> RD_COUNT_SHIFT) == 0)
 178                                 break;
 179                         rwlock->lock = val;
 180                 }
 181                 delay_backoff(iterations++);
 182                 val = __insn_tns((int *)&rwlock->lock);
 183         }
 184
 185         /* Take out the next ticket and extract my ticket value. */
 186         rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT);
 187         my_ticket_ = val >> WR_NEXT_SHIFT;
 188
 189         /* Wait until the "current" field matches our ticket. */
 190         for (;;) {
 191                 u32 curr_ = val >> WR_CURR_SHIFT;
 192                 u32 delta = ((my_ticket_ - curr_) & WR_MASK);
 193                 if (likely(delta == 0))
 194                         break;
 195
 196                 /* Delay based on how many lock-holders are still out there. */
 197                 relax((256 / CYCLES_PER_RELAX_LOOP) * delta);
 198
 199                 /*
 200                  * Get a non-tns value to check; we don't need to tns
 201                  * it ourselves.  Since we're not tns'ing, we retry
 202                  * more rapidly to get a valid value.
 203                  */
 204                 while ((val = rwlock->lock) & 1)
 205                         relax(4);
 206         }
 207 }
 208 EXPORT_SYMBOL(arch_write_lock);
 209
 210 int arch_write_trylock(arch_rwlock_t *rwlock)
 211 {
 212         u32 val = __insn_tns((int *)&rwlock->lock);
 213
 214         /*
 215          * If a tns is in progress, or there's a waiting or active locker,
 216          * or active readers, we can't take the lock, so give up.
 217          */
 218         if (unlikely(val != 0)) {
 219                 if (!(val & 1))
 220                         rwlock->lock = val;
 221                 return 0;
 222         }
 223
 224         /* Set the "next" field to mark it locked. */
 225         rwlock->lock = 1 << _WR_NEXT_SHIFT;
 226         return 1;
 227 }
 228 EXPORT_SYMBOL(arch_write_trylock);
 229
 230 void arch_write_unlock(arch_rwlock_t *rwlock)
 231 {
 232         u32 val, eq, mask;
 233
 234         mb();  /* guarantee anything modified under the lock is visible */
 235         val = __insn_tns((int *)&rwlock->lock);
 236         if (likely(val == (1 << _WR_NEXT_SHIFT))) {
 237                 rwlock->lock = 0;
 238                 return;
 239         }
 240         while (unlikely(val & 1)) {
 241                 /* Limited backoff since we are the highest-priority task. */
 242                 relax(4);
 243                 val = __insn_tns((int *)&rwlock->lock);
 244         }
 245         mask = 1 << WR_CURR_SHIFT;
 246         val = __insn_addb(val, mask);
 247         eq = __insn_seqb(val, val << (WR_CURR_SHIFT - WR_NEXT_SHIFT));
 248         val = __insn_mz(eq & mask, val);
 249         rwlock->lock = val;
 250 }
 251 EXPORT_SYMBOL(arch_write_unlock);