FreeBSD/Linux Kernel Cross Reference
sys/lib/rwsem-spinlock.c

     /* rwsem-spinlock.c: R/W semaphores: contention handling functions for generic spinlock
      *                                   implementation
      *
      * Copyright (c) 2001   David Howells (dhowells@redhat.com).
      * - Derived partially from idea by Andrea Arcangeli <andrea@suse.de>
      * - Derived also from comments by Linus
      *
      * Trylock by Brian Watson (Brian.J.Watson@compaq.com).
      */
    #include <linux/rwsem.h>
    #include <linux/sched.h>
    #include <linux/module.h>
    
    struct rwsem_waiter {
            struct list_head        list;
            struct task_struct      *task;
            unsigned int            flags;
    #define RWSEM_WAITING_FOR_READ  0x00000001
    #define RWSEM_WAITING_FOR_WRITE 0x00000002
    };
    
    #if RWSEM_DEBUG
    void rwsemtrace(struct rw_semaphore *sem, const char *str)
    {
            if (sem->debug)
                    printk("[%d] %s({%d,%d})\n",
                           current->pid,str,sem->activity,list_empty(&sem->wait_list)?0:1);
    }
    #endif
    
    /*
     * initialise the semaphore
     */
    void init_rwsem(struct rw_semaphore *sem)
    {
            sem->activity = 0;
            spin_lock_init(&sem->wait_lock);
            INIT_LIST_HEAD(&sem->wait_list);
    #if RWSEM_DEBUG
            sem->debug = 0;
    #endif
    }
    
    /*
     * handle the lock being released whilst there are processes blocked on it that can now run
     * - if we come here, then:
     *   - the 'active count' _reached_ zero
     *   - the 'waiting count' is non-zero
     * - the spinlock must be held by the caller
     * - woken process blocks are discarded from the list after having flags zeroised
     */
    static inline struct rw_semaphore *__rwsem_do_wake(struct rw_semaphore *sem)
    {
            struct rwsem_waiter *waiter;
            int woken;
    
            rwsemtrace(sem,"Entering __rwsem_do_wake");
    
            waiter = list_entry(sem->wait_list.next,struct rwsem_waiter,list);
    
            /* try to grant a single write lock if there's a writer at the front of the queue
             * - we leave the 'waiting count' incremented to signify potential contention
             */
            if (waiter->flags & RWSEM_WAITING_FOR_WRITE) {
                    sem->activity = -1;
                    list_del(&waiter->list);
                    waiter->flags = 0;
                    wake_up_process(waiter->task);
                    goto out;
            }
    
            /* grant an infinite number of read locks to the readers at the front of the queue */
            woken = 0;
            do {
                    list_del(&waiter->list);
                    waiter->flags = 0;
                    wake_up_process(waiter->task);
                    woken++;
                    if (list_empty(&sem->wait_list))
                            break;
                    waiter = list_entry(sem->wait_list.next,struct rwsem_waiter,list);
            } while (waiter->flags&RWSEM_WAITING_FOR_READ);
    
            sem->activity += woken;
    
     out:
            rwsemtrace(sem,"Leaving __rwsem_do_wake");
            return sem;
    }
    
    /*
     * wake a single writer
     */
    static inline struct rw_semaphore *__rwsem_wake_one_writer(struct rw_semaphore *sem)
    {
            struct rwsem_waiter *waiter;
    
            sem->activity = -1;
    
           waiter = list_entry(sem->wait_list.next,struct rwsem_waiter,list);
           list_del(&waiter->list);
   
           waiter->flags = 0;
           wake_up_process(waiter->task);
           return sem;
   }
   
   /*
    * get a read lock on the semaphore
    */
   void __down_read(struct rw_semaphore *sem)
   {
           struct rwsem_waiter waiter;
           struct task_struct *tsk;
   
           rwsemtrace(sem,"Entering __down_read");
   
           spin_lock(&sem->wait_lock);
   
           if (sem->activity>=0 && list_empty(&sem->wait_list)) {
                   /* granted */
                   sem->activity++;
                   spin_unlock(&sem->wait_lock);
                   goto out;
           }
   
           tsk = current;
           set_task_state(tsk,TASK_UNINTERRUPTIBLE);
   
           /* set up my own style of waitqueue */
           waiter.task = tsk;
           waiter.flags = RWSEM_WAITING_FOR_READ;
   
           list_add_tail(&waiter.list,&sem->wait_list);
   
           /* we don't need to touch the semaphore struct anymore */
           spin_unlock(&sem->wait_lock);
   
           /* wait to be given the lock */
           for (;;) {
                   if (!waiter.flags)
                           break;
                   schedule();
                   set_task_state(tsk, TASK_UNINTERRUPTIBLE);
           }
   
           tsk->state = TASK_RUNNING;
   
    out:
           rwsemtrace(sem,"Leaving __down_read");
   }
   
   /*
    * trylock for reading -- returns 1 if successful, 0 if contention
    */
   int __down_read_trylock(struct rw_semaphore *sem)
   {
           int ret = 0;
           rwsemtrace(sem,"Entering __down_read_trylock");
   
           spin_lock(&sem->wait_lock);
   
           if (sem->activity>=0 && list_empty(&sem->wait_list)) {
                   /* granted */
                   sem->activity++;
                   ret = 1;
           }
   
           spin_unlock(&sem->wait_lock);
   
           rwsemtrace(sem,"Leaving __down_read_trylock");
           return ret;
   }
   
   /*
    * get a write lock on the semaphore
    * - note that we increment the waiting count anyway to indicate an exclusive lock
    */
   void __down_write(struct rw_semaphore *sem)
   {
           struct rwsem_waiter waiter;
           struct task_struct *tsk;
   
           rwsemtrace(sem,"Entering __down_write");
   
           spin_lock(&sem->wait_lock);
   
           if (sem->activity==0 && list_empty(&sem->wait_list)) {
                   /* granted */
                   sem->activity = -1;
                   spin_unlock(&sem->wait_lock);
                   goto out;
           }
   
           tsk = current;
           set_task_state(tsk,TASK_UNINTERRUPTIBLE);
   
           /* set up my own style of waitqueue */
           waiter.task = tsk;
           waiter.flags = RWSEM_WAITING_FOR_WRITE;
   
           list_add_tail(&waiter.list,&sem->wait_list);
   
           /* we don't need to touch the semaphore struct anymore */
           spin_unlock(&sem->wait_lock);
   
           /* wait to be given the lock */
           for (;;) {
                   if (!waiter.flags)
                           break;
                   schedule();
                   set_task_state(tsk, TASK_UNINTERRUPTIBLE);
           }
   
           tsk->state = TASK_RUNNING;
   
    out:
           rwsemtrace(sem,"Leaving __down_write");
   }
   
   /*
    * trylock for writing -- returns 1 if successful, 0 if contention
    */
   int __down_write_trylock(struct rw_semaphore *sem)
   {
           int ret = 0;
           rwsemtrace(sem,"Entering __down_write_trylock");
   
           spin_lock(&sem->wait_lock);
   
           if (sem->activity==0 && list_empty(&sem->wait_list)) {
                   /* granted */
                   sem->activity = -1;
                   ret = 1;
           }
   
           spin_unlock(&sem->wait_lock);
   
           rwsemtrace(sem,"Leaving __down_write_trylock");
           return ret;
   }
   
   /*
    * release a read lock on the semaphore
    */
   void __up_read(struct rw_semaphore *sem)
   {
           rwsemtrace(sem,"Entering __up_read");
   
           spin_lock(&sem->wait_lock);
   
           if (--sem->activity==0 && !list_empty(&sem->wait_list))
                   sem = __rwsem_wake_one_writer(sem);
   
           spin_unlock(&sem->wait_lock);
   
           rwsemtrace(sem,"Leaving __up_read");
   }
   
   /*
    * release a write lock on the semaphore
    */
   void __up_write(struct rw_semaphore *sem)
   {
           rwsemtrace(sem,"Entering __up_write");
   
           spin_lock(&sem->wait_lock);
   
           sem->activity = 0;
           if (!list_empty(&sem->wait_list))
                   sem = __rwsem_do_wake(sem);
   
           spin_unlock(&sem->wait_lock);
   
           rwsemtrace(sem,"Leaving __up_write");
   }
   
   EXPORT_SYMBOL(init_rwsem);
   EXPORT_SYMBOL(__down_read);
   EXPORT_SYMBOL(__down_write);
   EXPORT_SYMBOL(__up_read);
   EXPORT_SYMBOL(__up_write);
   #if RWSEM_DEBUG
   EXPORT_SYMBOL(rwsemtrace);
   #endif

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