arch/parisc/kernel/semaphore.c

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