arch/sparc/kernel/semaphore.c

   1 /* $Id: semaphore.c,v 1.4 2000/11/10 04:02:03 davem Exp $
   2  * Generic semaphore code. Buyer beware. Do your own
   3  * specific changes in <asm/semaphore-helper.h>
   4  */
   5
   6 #include <linux/sched.h>
   7 #include <asm/semaphore-helper.h>
   8
   9 /*
  10  * Semaphores are implemented using a two-way counter:
  11  * The "count" variable is decremented for each process
  12  * that tries to sleep, while the "waking" variable is
  13  * incremented when the "up()" code goes to wake up waiting
  14  * processes.
  15  *
  16  * Notably, the inline "up()" and "down()" functions can
  17  * efficiently test if they need to do any extra work (up
  18  * needs to do something only if count was negative before
  19  * the increment operation.
  20  *
  21  * waking_non_zero() (from asm/semaphore.h) must execute
  22  * atomically.
  23  *
  24  * When __up() is called, the count was negative before
  25  * incrementing it, and we need to wake up somebody.
  26  *
  27  * This routine adds one to the count of processes that need to
  28  * wake up and exit.  ALL waiting processes actually wake up but
  29  * only the one that gets to the "waking" field first will gate
  30  * through and acquire the semaphore.  The others will go back
  31  * to sleep.
  32  *
  33  * Note that these functions are only called when there is
  34  * contention on the lock, and as such all this is the
  35  * "non-critical" part of the whole semaphore business. The
  36  * critical part is the inline stuff in <asm/semaphore.h>
  37  * where we want to avoid any extra jumps and calls.
  38  */
  39 void __up(struct semaphore *sem)
  40 {
  41         wake_one_more(sem);
  42         wake_up(&sem->wait);
  43 }
  44
  45 /*
  46  * Perform the "down" function.  Return zero for semaphore acquired,
  47  * return negative for signalled out of the function.
  48  *
  49  * If called from __down, the return is ignored and the wait loop is
  50  * not interruptible.  This means that a task waiting on a semaphore
  51  * using "down()" cannot be killed until someone does an "up()" on
  52  * the semaphore.
  53  *
  54  * If called from __down_interruptible, the return value gets checked
  55  * upon return.  If the return value is negative then the task continues
  56  * with the negative value in the return register (it can be tested by
  57  * the caller).
  58  *
  59  * Either form may be used in conjunction with "up()".
  60  *
  61  */
  62
  63 #define DOWN_VAR                                \
  64         struct task_struct *tsk = current;      \
  65         DECLARE_WAITQUEUE(wait, tsk);
  66
  67 #define DOWN_HEAD(task_state)                                           \
  68                                                                         \
  69                                                                         \
  70         tsk->state = (task_state);                                      \
  71         add_wait_queue(&sem->wait, &wait);                              \
  72                                                                         \
  73         /*                                                              \
  74          * Ok, we're set up.  sem->count is known to be less than zero  \
  75          * so we must wait.                                             \
  76          *                                                              \
  77          * We can let go the lock for purposes of waiting.              \
  78          * We re-acquire it after awaking so as to protect              \
  79          * all semaphore operations.                                    \
  80          *                                                              \
  81          * If "up()" is called before we call waking_non_zero() then    \
  82          * we will catch it right away.  If it is called later then     \
  83          * we will have to go through a wakeup cycle to catch it.       \
  84          *                                                              \
  85          * Multiple waiters contend for the semaphore lock to see       \
  86          * who gets to gate through and who has to wait some more.      \
  87          */                                                             \
  88         for (;;) {
  89
  90 #define DOWN_TAIL(task_state)                   \
  91                 tsk->state = (task_state);      \
  92         }                                       \
  93         tsk->state = TASK_RUNNING;              \
  94         remove_wait_queue(&sem->wait, &wait);
  95
  96 void __down(struct semaphore * sem)
  97 {
  98         DOWN_VAR
  99         DOWN_HEAD(TASK_UNINTERRUPTIBLE)
 100         if (waking_non_zero(sem))
 101                 break;
 102         schedule();
 103         DOWN_TAIL(TASK_UNINTERRUPTIBLE)
 104 }
 105
 106 int __down_interruptible(struct semaphore * sem)
 107 {
 108         int ret = 0;
 109         DOWN_VAR
 110         DOWN_HEAD(TASK_INTERRUPTIBLE)
 111
 112         ret = waking_non_zero_interruptible(sem, tsk);
 113         if (ret)
 114         {
 115                 if (ret == 1)
 116                         /* ret != 0 only if we get interrupted -arca */
 117                         ret = 0;
 118                 break;
 119         }
 120         schedule();
 121         DOWN_TAIL(TASK_INTERRUPTIBLE)
 122         return ret;
 123 }
 124
 125 int __down_trylock(struct semaphore * sem)
 126 {
 127         return waking_non_zero_trylock(sem);
 128 }
 129
 130 /* rw mutexes
 131  * Implemented by Jakub Jelinek (jakub@redhat.com) based on
 132  * i386 implementation by Ben LaHaise (bcrl@redhat.com).
 133  */
 134
 135 extern inline int ldstub(unsigned char *p)
 136 {
 137         int ret;
 138         asm volatile("ldstub %1, %0" : "=r" (ret) : "m" (*p) : "memory");
 139         return ret;
 140 }
 141
 142 void down_read_failed_biased(struct rw_semaphore *sem)
 143 {
 144         DOWN_VAR
 145
 146         add_wait_queue(&sem->wait, &wait);      /* put ourselves at the head of the list */
 147
 148         for (;;) {
 149                 if (!ldstub(&sem->read_not_granted))
 150                         break;
 151                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 152                 if (sem->read_not_granted)
 153                         schedule();
 154         }
 155
 156         remove_wait_queue(&sem->wait, &wait);
 157         tsk->state = TASK_RUNNING;
 158 }
 159
 160 void down_write_failed_biased(struct rw_semaphore *sem)
 161 {
 162         DOWN_VAR
 163
 164         add_wait_queue_exclusive(&sem->write_bias_wait, &wait); /* put ourselves at the end of the list */
 165
 166         for (;;) {
 167                 if (!ldstub(&sem->write_not_granted))
 168                         break;
 169                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 170                 if (sem->write_not_granted)
 171                         schedule();
 172         }
 173
 174         remove_wait_queue(&sem->write_bias_wait, &wait);
 175         tsk->state = TASK_RUNNING;
 176
 177         /* if the lock is currently unbiased, awaken the sleepers
 178          * FIXME: this wakes up the readers early in a bit of a
 179          * stampede -> bad!
 180          */
 181         if (sem->count >= 0)
 182                 wake_up(&sem->wait);
 183 }
 184
 185 /* Wait for the lock to become unbiased.  Readers
 186  * are non-exclusive. =)
 187  */
 188 void down_read_failed(struct rw_semaphore *sem)
 189 {
 190         DOWN_VAR
 191
 192         __up_read(sem); /* this takes care of granting the lock */
 193
 194         add_wait_queue(&sem->wait, &wait);
 195
 196         while (sem->count < 0) {
 197                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 198                 if (sem->count >= 0)
 199                         break;
 200                 schedule();
 201         }
 202
 203         remove_wait_queue(&sem->wait, &wait);
 204         tsk->state = TASK_RUNNING;
 205 }
 206
 207 /* Wait for the lock to become unbiased. Since we're
 208  * a writer, we'll make ourselves exclusive.
 209  */
 210 void down_write_failed(struct rw_semaphore *sem)
 211 {
 212         DOWN_VAR
 213
 214         __up_write(sem);        /* this takes care of granting the lock */
 215
 216         add_wait_queue_exclusive(&sem->wait, &wait);
 217
 218         while (sem->count < 0) {
 219                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 220                 if (sem->count >= 0)
 221                         break;  /* we must attempt to acquire or bias the lock */
 222                 schedule();
 223         }
 224
 225         remove_wait_queue(&sem->wait, &wait);
 226         tsk->state = TASK_RUNNING;
 227 }
 228
 229 void __rwsem_wake(struct rw_semaphore *sem, unsigned long readers)
 230 {
 231         if (readers) {
 232                 /* Due to lame ldstub we don't do here
 233                    a BUG() consistency check */
 234                 sem->read_not_granted = 0;
 235                 wake_up(&sem->wait);
 236         } else {
 237                 sem->write_not_granted = 0;
 238                 wake_up(&sem->write_bias_wait);
 239         }
 240 }