lib/rwsem-spinlock.c

   1 /* rwsem-spinlock.c: R/W semaphores: contention handling functions for
   2  * generic spinlock implementation
   3  *
   4  * Copyright (c) 2001   David Howells (dhowells@redhat.com).
   5  * - Derived partially from idea by Andrea Arcangeli <andrea@suse.de>
   6  * - Derived also from comments by Linus
   7  */
   8 #include <linux/rwsem.h>
   9 #include <linux/sched.h>
  10 #include <linux/export.h>
  11
  12 enum rwsem_waiter_type {
  13         RWSEM_WAITING_FOR_WRITE,
  14         RWSEM_WAITING_FOR_READ
  15 };
  16
  17 struct rwsem_waiter {
  18         struct list_head list;
  19         struct task_struct *task;
  20         enum rwsem_waiter_type type;
  21 };
  22
  23 int rwsem_is_locked(struct rw_semaphore *sem)
  24 {
  25         int ret = 1;
  26         unsigned long flags;
  27
  28         if (raw_spin_trylock_irqsave(&sem->wait_lock, flags)) {
  29                 ret = (sem->activity != 0);
  30                 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
  31         }
  32         return ret;
  33 }
  34 EXPORT_SYMBOL(rwsem_is_locked);
  35
  36 /*
  37  * initialise the semaphore
  38  */
  39 void __init_rwsem(struct rw_semaphore *sem, const char *name,
  40                   struct lock_class_key *key)
  41 {
  42 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  43         /*
  44          * Make sure we are not reinitializing a held semaphore:
  45          */
  46         debug_check_no_locks_freed((void *)sem, sizeof(*sem));
  47         lockdep_init_map(&sem->dep_map, name, key, 0);
  48 #endif
  49         sem->activity = 0;
  50         raw_spin_lock_init(&sem->wait_lock);
  51         INIT_LIST_HEAD(&sem->wait_list);
  52 }
  53 EXPORT_SYMBOL(__init_rwsem);
  54
  55 /*
  56  * handle the lock release when processes blocked on it that can now run
  57  * - if we come here, then:
  58  *   - the 'active count' _reached_ zero
  59  *   - the 'waiting count' is non-zero
  60  * - the spinlock must be held by the caller
  61  * - woken process blocks are discarded from the list after having task zeroed
  62  * - writers are only woken if wakewrite is non-zero
  63  */
  64 static inline struct rw_semaphore *
  65 __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
  66 {
  67         struct rwsem_waiter *waiter;
  68         struct task_struct *tsk;
  69         int woken;
  70
  71         waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
  72
  73         if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
  74                 if (wakewrite)
  75                         /* Wake up a writer. Note that we do not grant it the
  76                          * lock - it will have to acquire it when it runs. */
  77                         wake_up_process(waiter->task);
  78                 goto out;
  79         }
  80
  81         /* grant an infinite number of read locks to the front of the queue */
  82         woken = 0;
  83         do {
  84                 struct list_head *next = waiter->list.next;
  85
  86                 list_del(&waiter->list);
  87                 tsk = waiter->task;
  88                 smp_mb();
  89                 waiter->task = NULL;
  90                 wake_up_process(tsk);
  91                 put_task_struct(tsk);
  92                 woken++;
  93                 if (next == &sem->wait_list)
  94                         break;
  95                 waiter = list_entry(next, struct rwsem_waiter, list);
  96         } while (waiter->type != RWSEM_WAITING_FOR_WRITE);
  97
  98         sem->activity += woken;
  99
 100  out:
 101         return sem;
 102 }
 103
 104 /*
 105  * wake a single writer
 106  */
 107 static inline struct rw_semaphore *
 108 __rwsem_wake_one_writer(struct rw_semaphore *sem)
 109 {
 110         struct rwsem_waiter *waiter;
 111
 112         waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
 113         wake_up_process(waiter->task);
 114
 115         return sem;
 116 }
 117
 118 /*
 119  * get a read lock on the semaphore
 120  */
 121 void __sched __down_read(struct rw_semaphore *sem)
 122 {
 123         struct rwsem_waiter waiter;
 124         struct task_struct *tsk;
 125         unsigned long flags;
 126
 127         raw_spin_lock_irqsave(&sem->wait_lock, flags);
 128
 129         if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
 130                 /* granted */
 131                 sem->activity++;
 132                 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 133                 goto out;
 134         }
 135
 136         tsk = current;
 137         set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 138
 139         /* set up my own style of waitqueue */
 140         waiter.task = tsk;
 141         waiter.type = RWSEM_WAITING_FOR_READ;
 142         get_task_struct(tsk);
 143
 144         list_add_tail(&waiter.list, &sem->wait_list);
 145
 146         /* we don't need to touch the semaphore struct anymore */
 147         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 148
 149         /* wait to be given the lock */
 150         for (;;) {
 151                 if (!waiter.task)
 152                         break;
 153                 schedule();
 154                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 155         }
 156
 157         tsk->state = TASK_RUNNING;
 158  out:
 159         ;
 160 }
 161
 162 /*
 163  * trylock for reading -- returns 1 if successful, 0 if contention
 164  */
 165 int __down_read_trylock(struct rw_semaphore *sem)
 166 {
 167         unsigned long flags;
 168         int ret = 0;
 169
 170
 171         raw_spin_lock_irqsave(&sem->wait_lock, flags);
 172
 173         if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
 174                 /* granted */
 175                 sem->activity++;
 176                 ret = 1;
 177         }
 178
 179         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 180
 181         return ret;
 182 }
 183
 184 /*
 185  * get a write lock on the semaphore
 186  */
 187 void __sched __down_write_nested(struct rw_semaphore *sem, int subclass)
 188 {
 189         struct rwsem_waiter waiter;
 190         struct task_struct *tsk;
 191         unsigned long flags;
 192
 193         raw_spin_lock_irqsave(&sem->wait_lock, flags);
 194
 195         /* set up my own style of waitqueue */
 196         tsk = current;
 197         waiter.task = tsk;
 198         waiter.type = RWSEM_WAITING_FOR_WRITE;
 199         list_add_tail(&waiter.list, &sem->wait_list);
 200
 201         /* wait for someone to release the lock */
 202         for (;;) {
 203                 /*
 204                  * That is the key to support write lock stealing: allows the
 205                  * task already on CPU to get the lock soon rather than put
 206                  * itself into sleep and waiting for system woke it or someone
 207                  * else in the head of the wait list up.
 208                  */
 209                 if (sem->activity == 0)
 210                         break;
 211                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 212                 raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 213                 schedule();
 214                 raw_spin_lock_irqsave(&sem->wait_lock, flags);
 215         }
 216         /* got the lock */
 217         sem->activity = -1;
 218         list_del(&waiter.list);
 219
 220         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 221 }
 222
 223 void __sched __down_write(struct rw_semaphore *sem)
 224 {
 225         __down_write_nested(sem, 0);
 226 }
 227
 228 /*
 229  * trylock for writing -- returns 1 if successful, 0 if contention
 230  */
 231 int __down_write_trylock(struct rw_semaphore *sem)
 232 {
 233         unsigned long flags;
 234         int ret = 0;
 235
 236         raw_spin_lock_irqsave(&sem->wait_lock, flags);
 237
 238         if (sem->activity == 0) {
 239                 /* got the lock */
 240                 sem->activity = -1;
 241                 ret = 1;
 242         }
 243
 244         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 245
 246         return ret;
 247 }
 248
 249 /*
 250  * release a read lock on the semaphore
 251  */
 252 void __up_read(struct rw_semaphore *sem)
 253 {
 254         unsigned long flags;
 255
 256         raw_spin_lock_irqsave(&sem->wait_lock, flags);
 257
 258         if (--sem->activity == 0 && !list_empty(&sem->wait_list))
 259                 sem = __rwsem_wake_one_writer(sem);
 260
 261         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 262 }
 263
 264 /*
 265  * release a write lock on the semaphore
 266  */
 267 void __up_write(struct rw_semaphore *sem)
 268 {
 269         unsigned long flags;
 270
 271         raw_spin_lock_irqsave(&sem->wait_lock, flags);
 272
 273         sem->activity = 0;
 274         if (!list_empty(&sem->wait_list))
 275                 sem = __rwsem_do_wake(sem, 1);
 276
 277         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 278 }
 279
 280 /*
 281  * downgrade a write lock into a read lock
 282  * - just wake up any readers at the front of the queue
 283  */
 284 void __downgrade_write(struct rw_semaphore *sem)
 285 {
 286         unsigned long flags;
 287
 288         raw_spin_lock_irqsave(&sem->wait_lock, flags);
 289
 290         sem->activity = 1;
 291         if (!list_empty(&sem->wait_list))
 292                 sem = __rwsem_do_wake(sem, 0);
 293
 294         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 295 }
 296