arch/m68k/kernel/semaphore.c

   1 /*
   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
  64 #define DOWN_HEAD(task_state)                                           \
  65                                                                         \
  66                                                                         \
  67         current->state = (task_state);                                  \
  68         add_wait_queue(&sem->wait, &wait);                              \
  69                                                                         \
  70         /*                                                              \
  71          * Ok, we're set up.  sem->count is known to be less than zero  \
  72          * so we must wait.                                             \
  73          *                                                              \
  74          * We can let go the lock for purposes of waiting.              \
  75          * We re-acquire it after awaking so as to protect              \
  76          * all semaphore operations.                                    \
  77          *                                                              \
  78          * If "up()" is called before we call waking_non_zero() then    \
  79          * we will catch it right away.  If it is called later then     \
  80          * we will have to go through a wakeup cycle to catch it.       \
  81          *                                                              \
  82          * Multiple waiters contend for the semaphore lock to see       \
  83          * who gets to gate through and who has to wait some more.      \
  84          */                                                             \
  85         for (;;) {
  86
  87 #define DOWN_TAIL(task_state)                   \
  88                 current->state = (task_state);  \
  89         }                                       \
  90         current->state = TASK_RUNNING;          \
  91         remove_wait_queue(&sem->wait, &wait);
  92
  93 void __down(struct semaphore * sem)
  94 {
  95         DECLARE_WAITQUEUE(wait, current);
  96
  97         DOWN_HEAD(TASK_UNINTERRUPTIBLE)
  98         if (waking_non_zero(sem))
  99                 break;
 100         schedule();
 101         DOWN_TAIL(TASK_UNINTERRUPTIBLE)
 102 }
 103
 104 int __down_interruptible(struct semaphore * sem)
 105 {
 106         DECLARE_WAITQUEUE(wait, current);
 107         int ret = 0;
 108
 109         DOWN_HEAD(TASK_INTERRUPTIBLE)
 110
 111         ret = waking_non_zero_interruptible(sem, current);
 112         if (ret)
 113         {
 114                 if (ret == 1)
 115                         /* ret != 0 only if we get interrupted -arca */
 116                         ret = 0;
 117                 break;
 118         }
 119         schedule();
 120         DOWN_TAIL(TASK_INTERRUPTIBLE)
 121         return ret;
 122 }
 123
 124 int __down_trylock(struct semaphore * sem)
 125 {
 126         return waking_non_zero_trylock(sem);
 127 }
 128
 129
 130 /* Wait for the lock to become unbiased.  Readers
 131  * are non-exclusive. =)
 132  */
 133 void down_read_failed(struct rw_semaphore *sem)
 134 {
 135         DECLARE_WAITQUEUE(wait, current);
 136
 137         __up_read(sem); /* this takes care of granting the lock */
 138
 139         add_wait_queue(&sem->wait, &wait);
 140
 141         while (atomic_read(&sem->count) < 0) {
 142                 set_task_state(current, TASK_UNINTERRUPTIBLE);
 143                 if (atomic_read(&sem->count) >= 0)
 144                         break;
 145                 schedule();
 146         }
 147
 148         remove_wait_queue(&sem->wait, &wait);
 149         current->state = TASK_RUNNING;
 150 }
 151
 152 void down_read_failed_biased(struct rw_semaphore *sem)
 153 {
 154         DECLARE_WAITQUEUE(wait, current);
 155
 156         add_wait_queue(&sem->wait, &wait);      /* put ourselves at the head of the list */
 157
 158         for (;;) {
 159                 if (sem->read_bias_granted && xchg(&sem->read_bias_granted, 0))
 160                         break;
 161                 set_task_state(current, TASK_UNINTERRUPTIBLE);
 162                 if (!sem->read_bias_granted)
 163                         schedule();
 164         }
 165
 166         remove_wait_queue(&sem->wait, &wait);
 167         current->state = TASK_RUNNING;
 168 }
 169
 170
 171 /* Wait for the lock to become unbiased. Since we're
 172  * a writer, we'll make ourselves exclusive.
 173  */
 174 void down_write_failed(struct rw_semaphore *sem)
 175 {
 176         DECLARE_WAITQUEUE(wait, current);
 177
 178         __up_write(sem);        /* this takes care of granting the lock */
 179
 180         add_wait_queue_exclusive(&sem->wait, &wait);
 181
 182         while (atomic_read(&sem->count) < 0) {
 183                 set_task_state(current, TASK_UNINTERRUPTIBLE);
 184                 if (atomic_read(&sem->count) >= 0)
 185                         break;  /* we must attempt to acquire or bias the lock */
 186                 schedule();
 187         }
 188
 189         remove_wait_queue(&sem->wait, &wait);
 190         current->state = TASK_RUNNING;
 191 }
 192
 193 void down_write_failed_biased(struct rw_semaphore *sem)
 194 {
 195         DECLARE_WAITQUEUE(wait, current);
 196
 197         add_wait_queue_exclusive(&sem->write_bias_wait, &wait); /* put ourselves at the end of the list */
 198
 199         for (;;) {
 200                 if (sem->write_bias_granted && xchg(&sem->write_bias_granted, 0))
 201                         break;
 202                 set_task_state(current, TASK_UNINTERRUPTIBLE);
 203                 if (!sem->write_bias_granted)
 204                         schedule();
 205         }
 206
 207         remove_wait_queue(&sem->write_bias_wait, &wait);
 208         current->state = TASK_RUNNING;
 209
 210         /* if the lock is currently unbiased, awaken the sleepers
 211          * FIXME: this wakes up the readers early in a bit of a
 212          * stampede -> bad!
 213          */
 214         if (atomic_read(&sem->count) >= 0)
 215                 wake_up(&sem->wait);
 216 }
 217
 218
 219 /* Called when someone has done an up that transitioned from
 220  * negative to non-negative, meaning that the lock has been
 221  * granted to whomever owned the bias.
 222  */
 223 void rwsem_wake_readers(struct rw_semaphore *sem)
 224 {
 225         if (xchg(&sem->read_bias_granted, 1))
 226                 BUG();
 227         wake_up(&sem->wait);
 228 }
 229
 230 void rwsem_wake_writer(struct rw_semaphore *sem)
 231 {
 232         if (xchg(&sem->write_bias_granted, 1))
 233                 BUG();
 234         wake_up(&sem->write_bias_wait);
 235 }