lib/rwsem.c

   1 /* rwsem.c: R/W semaphores: contention handling functions
   2  *
   3  * Written by David Howells (dhowells@redhat.com).
   4  * Derived from arch/i386/kernel/semaphore.c
   5  */
   6 #include <linux/rwsem.h>
   7 #include <linux/sched.h>
   8 #include <linux/init.h>
   9 #include <linux/module.h>
  10
  11 /*
  12  * Initialize an rwsem:
  13  */
  14 void __init_rwsem(struct rw_semaphore *sem, const char *name,
  15                   struct lock_class_key *key)
  16 {
  17 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  18         /*
  19          * Make sure we are not reinitializing a held semaphore:
  20          */
  21         debug_check_no_locks_freed((void *)sem, sizeof(*sem));
  22         lockdep_init_map(&sem->dep_map, name, key, 0);
  23 #endif
  24         sem->count = RWSEM_UNLOCKED_VALUE;
  25         spin_lock_init(&sem->wait_lock);
  26         INIT_LIST_HEAD(&sem->wait_list);
  27 }
  28
  29 EXPORT_SYMBOL(__init_rwsem);
  30
  31 struct rwsem_waiter {
  32         struct list_head list;
  33         struct task_struct *task;
  34         unsigned int flags;
  35 #define RWSEM_WAITING_FOR_READ  0x00000001
  36 #define RWSEM_WAITING_FOR_WRITE 0x00000002
  37 };
  38
  39 /*
  40  * handle the lock release when processes blocked on it that can now run
  41  * - if we come here from up_xxxx(), then:
  42  *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
  43  *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
  44  *   - there must be someone on the queue
  45  * - the spinlock must be held by the caller
  46  * - woken process blocks are discarded from the list after having task zeroed
  47  * - writers are only woken if downgrading is false
  48  */
  49 static inline struct rw_semaphore *
  50 __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
  51 {
  52         struct rwsem_waiter *waiter;
  53         struct task_struct *tsk;
  54         struct list_head *next;
  55         signed long oldcount, woken, loop;
  56
  57         if (downgrading)
  58                 goto dont_wake_writers;
  59
  60         /* if we came through an up_xxxx() call, we only only wake someone up
  61          * if we can transition the active part of the count from 0 -> 1
  62          */
  63  try_again:
  64         oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
  65                                                 - RWSEM_ACTIVE_BIAS;
  66         if (oldcount & RWSEM_ACTIVE_MASK)
  67                 goto undo;
  68
  69         waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
  70
  71         /* try to grant a single write lock if there's a writer at the front
  72          * of the queue - note we leave the 'active part' of the count
  73          * incremented by 1 and the waiting part incremented by 0x00010000
  74          */
  75         if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
  76                 goto readers_only;
  77
  78         /* We must be careful not to touch 'waiter' after we set ->task = NULL.
  79          * It is an allocated on the waiter's stack and may become invalid at
  80          * any time after that point (due to a wakeup from another source).
  81          */
  82         list_del(&waiter->list);
  83         tsk = waiter->task;
  84         smp_mb();
  85         waiter->task = NULL;
  86         wake_up_process(tsk);
  87         put_task_struct(tsk);
  88         goto out;
  89
  90         /* don't want to wake any writers */
  91  dont_wake_writers:
  92         waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
  93         if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
  94                 goto out;
  95
  96         /* grant an infinite number of read locks to the readers at the front
  97          * of the queue
  98          * - note we increment the 'active part' of the count by the number of
  99          *   readers before waking any processes up
 100          */
 101  readers_only:
 102         woken = 0;
 103         do {
 104                 woken++;
 105
 106                 if (waiter->list.next == &sem->wait_list)
 107                         break;
 108
 109                 waiter = list_entry(waiter->list.next,
 110                                         struct rwsem_waiter, list);
 111
 112         } while (waiter->flags & RWSEM_WAITING_FOR_READ);
 113
 114         loop = woken;
 115         woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS;
 116         if (!downgrading)
 117                 /* we'd already done one increment earlier */
 118                 woken -= RWSEM_ACTIVE_BIAS;
 119
 120         rwsem_atomic_add(woken, sem);
 121
 122         next = sem->wait_list.next;
 123         for (; loop > 0; loop--) {
 124                 waiter = list_entry(next, struct rwsem_waiter, list);
 125                 next = waiter->list.next;
 126                 tsk = waiter->task;
 127                 smp_mb();
 128                 waiter->task = NULL;
 129                 wake_up_process(tsk);
 130                 put_task_struct(tsk);
 131         }
 132
 133         sem->wait_list.next = next;
 134         next->prev = &sem->wait_list;
 135
 136  out:
 137         return sem;
 138
 139         /* undo the change to the active count, but check for a transition
 140          * 1->0 */
 141  undo:
 142         if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
 143                 goto out;
 144         goto try_again;
 145 }
 146
 147 /*
 148  * wait for a lock to be granted
 149  */
 150 static struct rw_semaphore __sched *
 151 rwsem_down_failed_common(struct rw_semaphore *sem,
 152                         struct rwsem_waiter *waiter, signed long adjustment)
 153 {
 154         struct task_struct *tsk = current;
 155         signed long count;
 156
 157         set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 158
 159         /* set up my own style of waitqueue */
 160         spin_lock_irq(&sem->wait_lock);
 161         waiter->task = tsk;
 162         get_task_struct(tsk);
 163
 164         list_add_tail(&waiter->list, &sem->wait_list);
 165
 166         /* we're now waiting on the lock, but no longer actively read-locking */
 167         count = rwsem_atomic_update(adjustment, sem);
 168
 169         /* if there are no active locks, wake the front queued process(es) up */
 170         if (!(count & RWSEM_ACTIVE_MASK))
 171                 sem = __rwsem_do_wake(sem, 0);
 172
 173         spin_unlock_irq(&sem->wait_lock);
 174
 175         /* wait to be given the lock */
 176         for (;;) {
 177                 if (!waiter->task)
 178                         break;
 179                 schedule();
 180                 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 181         }
 182
 183         tsk->state = TASK_RUNNING;
 184
 185         return sem;
 186 }
 187
 188 /*
 189  * wait for the read lock to be granted
 190  */
 191 asmregparm struct rw_semaphore __sched *
 192 rwsem_down_read_failed(struct rw_semaphore *sem)
 193 {
 194         struct rwsem_waiter waiter;
 195
 196         waiter.flags = RWSEM_WAITING_FOR_READ;
 197         rwsem_down_failed_common(sem, &waiter,
 198                                 RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
 199         return sem;
 200 }
 201
 202 /*
 203  * wait for the write lock to be granted
 204  */
 205 asmregparm struct rw_semaphore __sched *
 206 rwsem_down_write_failed(struct rw_semaphore *sem)
 207 {
 208         struct rwsem_waiter waiter;
 209
 210         waiter.flags = RWSEM_WAITING_FOR_WRITE;
 211         rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
 212
 213         return sem;
 214 }
 215
 216 /*
 217  * handle waking up a waiter on the semaphore
 218  * - up_read/up_write has decremented the active part of count if we come here
 219  */
 220 asmregparm struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
 221 {
 222         unsigned long flags;
 223
 224         spin_lock_irqsave(&sem->wait_lock, flags);
 225
 226         /* do nothing if list empty */
 227         if (!list_empty(&sem->wait_list))
 228                 sem = __rwsem_do_wake(sem, 0);
 229
 230         spin_unlock_irqrestore(&sem->wait_lock, flags);
 231
 232         return sem;
 233 }
 234
 235 /*
 236  * downgrade a write lock into a read lock
 237  * - caller incremented waiting part of count and discovered it still negative
 238  * - just wake up any readers at the front of the queue
 239  */
 240 asmregparm struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
 241 {
 242         unsigned long flags;
 243
 244         spin_lock_irqsave(&sem->wait_lock, flags);
 245
 246         /* do nothing if list empty */
 247         if (!list_empty(&sem->wait_list))
 248                 sem = __rwsem_do_wake(sem, 1);
 249
 250         spin_unlock_irqrestore(&sem->wait_lock, flags);
 251
 252         return sem;
 253 }
 254
 255 EXPORT_SYMBOL(rwsem_down_read_failed);
 256 EXPORT_SYMBOL(rwsem_down_write_failed);
 257 EXPORT_SYMBOL(rwsem_wake);
 258 EXPORT_SYMBOL(rwsem_downgrade_wake);