arch/xtensa/kernel/semaphore.c

   1 /*
   2  * arch/xtensa/kernel/semaphore.c
   3  *
   4  * Generic semaphore code. Buyer beware. Do your own specific changes
   5  * in <asm/semaphore-helper.h>
   6  *
   7  * This file is subject to the terms and conditions of the GNU General Public
   8  * License.  See the file "COPYING" in the main directory of this archive
   9  * for more details.
  10  *
  11  * Copyright (C) 2001 - 2005 Tensilica Inc.
  12  *
  13  * Joe Taylor   <joe@tensilica.com, joetylr@yahoo.com>
  14  * Chris Zankel <chris@zankel.net>
  15  * Marc Gauthier<marc@tensilica.com, marc@alumni.uwaterloo.ca>
  16  * Kevin Chea
  17  */
  18
  19 #include <linux/sched.h>
  20 #include <linux/wait.h>
  21 #include <linux/init.h>
  22 #include <asm/semaphore.h>
  23 #include <asm/errno.h>
  24
  25 /*
  26  * These two _must_ execute atomically wrt each other.
  27  */
  28
  29 static __inline__ void wake_one_more(struct semaphore * sem)
  30 {
  31         atomic_inc((atomic_t *)&sem->sleepers);
  32 }
  33
  34 static __inline__ int waking_non_zero(struct semaphore *sem)
  35 {
  36         unsigned long flags;
  37         int ret = 0;
  38
  39         spin_lock_irqsave(&semaphore_wake_lock, flags);
  40         if (sem->sleepers > 0) {
  41                 sem->sleepers--;
  42                 ret = 1;
  43         }
  44         spin_unlock_irqrestore(&semaphore_wake_lock, flags);
  45         return ret;
  46 }
  47
  48 /*
  49  * waking_non_zero_interruptible:
  50  *      1       got the lock
  51  *      0       go to sleep
  52  *      -EINTR  interrupted
  53  *
  54  * We must undo the sem->count down_interruptible() increment while we are
  55  * protected by the spinlock in order to make atomic this atomic_inc() with the
  56  * atomic_read() in wake_one_more(), otherwise we can race. -arca
  57  */
  58
  59 static __inline__ int waking_non_zero_interruptible(struct semaphore *sem,
  60                                                 struct task_struct *tsk)
  61 {
  62         unsigned long flags;
  63         int ret = 0;
  64
  65         spin_lock_irqsave(&semaphore_wake_lock, flags);
  66         if (sem->sleepers > 0) {
  67                 sem->sleepers--;
  68                 ret = 1;
  69         } else if (signal_pending(tsk)) {
  70                 atomic_inc(&sem->count);
  71                 ret = -EINTR;
  72         }
  73         spin_unlock_irqrestore(&semaphore_wake_lock, flags);
  74         return ret;
  75 }
  76
  77 /*
  78  * waking_non_zero_trylock:
  79  *      1       failed to lock
  80  *      0       got the lock
  81  *
  82  * We must undo the sem->count down_trylock() increment while we are
  83  * protected by the spinlock in order to make atomic this atomic_inc() with the
  84  * atomic_read() in wake_one_more(), otherwise we can race. -arca
  85  */
  86
  87 static __inline__ int waking_non_zero_trylock(struct semaphore *sem)
  88 {
  89         unsigned long flags;
  90         int ret = 1;
  91
  92         spin_lock_irqsave(&semaphore_wake_lock, flags);
  93         if (sem->sleepers <= 0)
  94                 atomic_inc(&sem->count);
  95         else {
  96                 sem->sleepers--;
  97                 ret = 0;
  98         }
  99         spin_unlock_irqrestore(&semaphore_wake_lock, flags);
 100         return ret;
 101 }
 102
 103 DEFINE_SPINLOCK(semaphore_wake_lock);
 104
 105 /*
 106  * Semaphores are implemented using a two-way counter:
 107  * The "count" variable is decremented for each process
 108  * that tries to sleep, while the "waking" variable is
 109  * incremented when the "up()" code goes to wake up waiting
 110  * processes.
 111  *
 112  * Notably, the inline "up()" and "down()" functions can
 113  * efficiently test if they need to do any extra work (up
 114  * needs to do something only if count was negative before
 115  * the increment operation.
 116  *
 117  * waking_non_zero() (from asm/semaphore.h) must execute
 118  * atomically.
 119  *
 120  * When __up() is called, the count was negative before
 121  * incrementing it, and we need to wake up somebody.
 122  *
 123  * This routine adds one to the count of processes that need to
 124  * wake up and exit.  ALL waiting processes actually wake up but
 125  * only the one that gets to the "waking" field first will gate
 126  * through and acquire the semaphore.  The others will go back
 127  * to sleep.
 128  *
 129  * Note that these functions are only called when there is
 130  * contention on the lock, and as such all this is the
 131  * "non-critical" part of the whole semaphore business. The
 132  * critical part is the inline stuff in <asm/semaphore.h>
 133  * where we want to avoid any extra jumps and calls.
 134  */
 135
 136 void __up(struct semaphore *sem)
 137 {
 138         wake_one_more(sem);
 139         wake_up(&sem->wait);
 140 }
 141
 142 /*
 143  * Perform the "down" function.  Return zero for semaphore acquired,
 144  * return negative for signalled out of the function.
 145  *
 146  * If called from __down, the return is ignored and the wait loop is
 147  * not interruptible.  This means that a task waiting on a semaphore
 148  * using "down()" cannot be killed until someone does an "up()" on
 149  * the semaphore.
 150  *
 151  * If called from __down_interruptible, the return value gets checked
 152  * upon return.  If the return value is negative then the task continues
 153  * with the negative value in the return register (it can be tested by
 154  * the caller).
 155  *
 156  * Either form may be used in conjunction with "up()".
 157  *
 158  */
 159
 160 #define DOWN_VAR                                \
 161         struct task_struct *tsk = current;      \
 162         wait_queue_t wait;                      \
 163         init_waitqueue_entry(&wait, tsk);
 164
 165 #define DOWN_HEAD(task_state)                                           \
 166                                                                         \
 167                                                                         \
 168         tsk->state = (task_state);                                      \
 169         add_wait_queue(&sem->wait, &wait);                              \
 170                                                                         \
 171         /*                                                              \
 172          * Ok, we're set up.  sem->count is known to be less than zero  \
 173          * so we must wait.                                             \
 174          *                                                              \
 175          * We can let go the lock for purposes of waiting.              \
 176          * We re-acquire it after awaking so as to protect              \
 177          * all semaphore operations.                                    \
 178          *                                                              \
 179          * If "up()" is called before we call waking_non_zero() then    \
 180          * we will catch it right away.  If it is called later then     \
 181          * we will have to go through a wakeup cycle to catch it.       \
 182          *                                                              \
 183          * Multiple waiters contend for the semaphore lock to see       \
 184          * who gets to gate through and who has to wait some more.      \
 185          */                                                             \
 186         for (;;) {
 187
 188 #define DOWN_TAIL(task_state)                   \
 189                 tsk->state = (task_state);      \
 190         }                                       \
 191         tsk->state = TASK_RUNNING;              \
 192         remove_wait_queue(&sem->wait, &wait);
 193
 194 void __sched __down(struct semaphore * sem)
 195 {
 196         DOWN_VAR
 197         DOWN_HEAD(TASK_UNINTERRUPTIBLE)
 198         if (waking_non_zero(sem))
 199                 break;
 200         schedule();
 201         DOWN_TAIL(TASK_UNINTERRUPTIBLE)
 202 }
 203
 204 int __sched __down_interruptible(struct semaphore * sem)
 205 {
 206         int ret = 0;
 207         DOWN_VAR
 208         DOWN_HEAD(TASK_INTERRUPTIBLE)
 209
 210         ret = waking_non_zero_interruptible(sem, tsk);
 211         if (ret)
 212         {
 213                 if (ret == 1)
 214                         /* ret != 0 only if we get interrupted -arca */
 215                         ret = 0;
 216                 break;
 217         }
 218         schedule();
 219         DOWN_TAIL(TASK_INTERRUPTIBLE)
 220         return ret;
 221 }
 222
 223 int __down_trylock(struct semaphore * sem)
 224 {
 225         return waking_non_zero_trylock(sem);
 226 }