spinlock.c

Axis 221 camera embedded programing interface

/* Linuxthreads - a simple clone()-based implementation of Posix        */
/* threads for Linux.                                                   */
/* Copyright (C) 1998 Xavier Leroy ([e-mail])              */
/*                                                                      */
/* This program is free software; you can redistribute it and/or        */
/* modify it under the terms of the GNU Library General Public License  */
/* as published by the Free Software Foundation; either version 2       */
/* of the License, or (at your option) any later version.               */
/*                                                                      */
/* This program is distributed in the hope that it will be useful,      */
/* but WITHOUT ANY WARRANTY; without even the implied warranty of       */
/* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the        */
/* GNU Library General Public License for more details.                 */

/* Internal locks */

#define __FORCE_GLIBC
#include <features.h>
#include <errno.h>
#include <sched.h>
#include <time.h>
#include <stdlib.h>
#include <limits.h>
#include "pthread.h"
#include "internals.h"
#include "spinlock.h"
#include "restart.h"

static void __pthread_acquire(int * spinlock);

static inline void __pthread_release(int * spinlock)
{
  WRITE_MEMORY_BARRIER();
  *spinlock = __LT_SPINLOCK_INIT;
  __asm __volatile ("" : "=m" (*spinlock) : "0" (*spinlock));
}


/* The status field of a spinlock is a pointer whose least significant
   bit is a locked flag.

   Thus the field values have the following meanings:

   status == 0:       spinlock is free
   status == 1:       spinlock is taken; no thread is waiting on it

   (status & 1) == 1: spinlock is taken and (status & ~1L) is a
                      pointer to the first waiting thread; other
		      waiting threads are linked via the p_nextlock
		      field.
   (status & 1) == 0: same as above, but spinlock is not taken.

   The waiting list is not sorted by priority order.
   Actually, we always insert at top of list (sole insertion mode
   that can be performed without locking).
   For __pthread_unlock, we perform a linear search in the list
   to find the highest-priority, oldest waiting thread.
   This is safe because there are no concurrent __pthread_unlock
   operations -- only the thread that locked the mutex can unlock it. */


void internal_function __pthread_lock(struct _pthread_fastlock * lock,
				      pthread_descr self)
{
#if defined HAS_COMPARE_AND_SWAP
  long oldstatus, newstatus;
  int successful_seizure, spurious_wakeup_count;
  int spin_count;
#endif

#if defined TEST_FOR_COMPARE_AND_SWAP
  if (!__pthread_has_cas)
#endif
#if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP
  {
    __pthread_acquire(&lock->__spinlock);
    return;
  }
#endif

#if defined HAS_COMPARE_AND_SWAP
  /* First try it without preparation.  Maybe it's a completely
     uncontested lock.  */
  if (lock->__status == 0 && __compare_and_swap (&lock->__status, 0, 1))
    return;

  spurious_wakeup_count = 0;
  spin_count = 0;

  /* On SMP, try spinning to get the lock. */
#if 0
  if (__pthread_smp_kernel) {
    int max_count = lock->__spinlock * 2 + 10;

    if (max_count > MAX_ADAPTIVE_SPIN_COUNT)
      max_count = MAX_ADAPTIVE_SPIN_COUNT;

    for (spin_count = 0; spin_count < max_count; spin_count++) {
      if (((oldstatus = lock->__status) & 1) == 0) {
	if(__compare_and_swap(&lock->__status, oldstatus, oldstatus | 1))
	{
	  if (spin_count)
	    lock->__spinlock += (spin_count - lock->__spinlock) / 8;
	  READ_MEMORY_BARRIER();
	  return;
	}
      }
#ifdef BUSY_WAIT_NOP
      BUSY_WAIT_NOP;
#endif
      __asm __volatile ("" : "=m" (lock->__status) : "0" (lock->__status));
    }

    lock->__spinlock += (spin_count - lock->__spinlock) / 8;
  }
#endif	

again:

  /* No luck, try once more or suspend. */

  do {
    oldstatus = lock->__status;
    successful_seizure = 0;

    if ((oldstatus & 1) == 0) {
      newstatus = oldstatus | 1;
      successful_seizure = 1;
    } else {
      if (self == NULL)
	self = thread_self();
      newstatus = (long) self | 1;
    }

    if (self != NULL) {
      THREAD_SETMEM(self, p_nextlock, (pthread_descr) (oldstatus));
      /* Make sure the store in p_nextlock completes before performing
         the compare-and-swap */
      MEMORY_BARRIER();
    }
  } while(! __compare_and_swap(&lock->__status, oldstatus, newstatus));

  /* Suspend with guard against spurious wakeup.
     This can happen in pthread_cond_timedwait_relative, when the thread
     wakes up due to timeout and is still on the condvar queue, and then
     locks the queue to remove itself. At that point it may still be on the
     queue, and may be resumed by a condition signal. */

  if (!successful_seizure) {
    for (;;) {
      suspend(self);
      if (self->p_nextlock != NULL) {
	/* Count resumes that don't belong to us. */
	spurious_wakeup_count++;
	continue;
      }
      break;
    }
    goto again;
  }

  /* Put back any resumes we caught that don't belong to us. */
  while (spurious_wakeup_count--)
    restart(self);

  READ_MEMORY_BARRIER();
#endif
}

int __pthread_unlock(struct _pthread_fastlock * lock)
{
#if defined HAS_COMPARE_AND_SWAP
  long oldstatus;
  pthread_descr thr, * ptr, * maxptr;
  int maxprio;
#endif

#if defined TEST_FOR_COMPARE_AND_SWAP
  if (!__pthread_has_cas)
#endif
#if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP
  {
    __pthread_release(&lock->__spinlock);
    return 0;
  }
#endif

#if defined HAS_COMPARE_AND_SWAP
  WRITE_MEMORY_BARRIER();

again:
  while ((oldstatus = lock->__status) == 1) {
    if (__compare_and_swap_with_release_semantics(&lock->__status,
	oldstatus, 0))
      return 0;
  }

  /* Find thread in waiting queue with maximal priority */
  ptr = (pthread_descr *) &lock->__status;
  thr = (pthread_descr) (oldstatus & ~1L);
  maxprio = 0;
  maxptr = ptr;

  /* Before we iterate over the wait queue, we need to execute
     a read barrier, otherwise we may read stale contents of nodes that may
     just have been inserted by other processors. One read barrier is enough to
     ensure we have a stable list; we don't need one for each pointer chase
     through the list, because we are the owner of the lock; other threads
     can only add nodes at the front; if a front node is consistent,
     the ones behind it must also be. */

  READ_MEMORY_BARRIER();

  while (thr != 0) {
    if (thr->p_priority >= maxprio) {
      maxptr = ptr;
      maxprio = thr->p_priority;
    }
    ptr = &(thr->p_nextlock);
    thr = (pthread_descr)((long)(thr->p_nextlock) & ~1L);
  }

  /* Remove max prio thread from waiting list. */
  if (maxptr == (pthread_descr *) &lock->__status) {
    /* If max prio thread is at head, remove it with compare-and-swap
       to guard against concurrent lock operation. This removal
       also has the side effect of marking the lock as released
       because the new status comes from thr->p_nextlock whose
       least significant bit is clear. */
    thr = (pthread_descr) (oldstatus & ~1L);
    if (! __compare_and_swap_with_release_semantics
	    (&lock->__status, oldstatus, (long)(thr->p_nextlock) & ~1L))
      goto again;
  } else {
    /* No risk of concurrent access, remove max prio thread normally.
       But in this case we must also flip the least significant bit
       of the status to mark the lock as released. */
    thr = (pthread_descr)((long)*maxptr & ~1L);
    *maxptr = thr->p_nextlock;

    /* Ensure deletion from linked list completes before we
       release the lock. */
    WRITE_MEMORY_BARRIER();

    do {
      oldstatus = lock->__status;
    } while (!__compare_and_swap_with_release_semantics(&lock->__status,
	     oldstatus, oldstatus & ~1L));
  }

  /* Wake up the selected waiting thread. Woken thread can check
     its own p_nextlock field for NULL to detect that it has been removed. No
     barrier is needed here, since restart() and suspend() take
     care of memory synchronization. */

  thr->p_nextlock = NULL;
  restart(thr);

  return 0;
#endif
}

/*
 * Alternate fastlocks do not queue threads directly. Instead, they queue
 * these wait queue node structures. When a timed wait wakes up due to
 * a timeout, it can leave its wait node in the queue (because there
 * is no safe way to remove from the quue). Some other thread will
 * deallocate the abandoned node.
 */


struct wait_node {
  struct wait_node *next;	/* Next node in null terminated linked list */
  pthread_descr thr;		/* The thread waiting with this node */
  int abandoned;		/* Atomic flag */
};

static long wait_node_free_list;
static int wait_node_free_list_spinlock;

/* Allocate a new node from the head of the free list using an atomic
   operation, or else using malloc if that list is empty.  A fundamental
   assumption here is that we can safely access wait_node_free_list->next.
   That's because we never free nodes once we allocate them, so a pointer to a
   node remains valid indefinitely. */

static struct wait_node *wait_node_alloc(void)
{
    struct wait_node *new_node = 0;

    __pthread_acquire(&wait_node_free_list_spinlock);
    if (wait_node_free_list != 0) {
      new_node = (struct wait_node *) wait_node_free_list;
      wait_node_free_list = (long) new_node->next;
    }
    WRITE_MEMORY_BARRIER();
    __pthread_release(&wait_node_free_list_spinlock);

    if (new_node == 0)
      return malloc(sizeof *wait_node_alloc());

    return new_node;
}

/* Return a node to the head of the free list using an atomic
   operation. */

static void wait_node_free(struct wait_node *wn)
{
    __pthread_acquire(&wait_node_free_list_spinlock);
    wn->next = (struct wait_node *) wait_node_free_list;
    wait_node_free_list = (long) wn;
    WRITE_MEMORY_BARRIER();
    __pthread_release(&wait_node_free_list_spinlock);
    return;
}

#if defined HAS_COMPARE_AND_SWAP

/* Remove a wait node from the specified queue.  It is assumed
   that the removal takes place concurrently with only atomic insertions at the
   head of the queue. */

static void wait_node_dequeue(struct wait_node **pp_head,
			      struct wait_node **pp_node,
			      struct wait_node *p_node)
{
  /* If the node is being deleted from the head of the
     list, it must be deleted using atomic compare-and-swap.
     Otherwise it can be deleted in the straightforward way. */

  if (pp_node == pp_head) {
    /* We don't need a read barrier between these next two loads,
       because it is assumed that the caller has already ensured
       the stability of *p_node with respect to p_node. */

    long oldvalue = (long) p_node;
    long newvalue = (long) p_node->next;

    if (__compare_and_swap((long *) pp_node, oldvalue, newvalue))
      return;

    /* Oops! Compare and swap failed, which means the node is
       no longer first. We delete it using the ordinary method.  But we don't
       know the identity of the node which now holds the pointer to the node
       being deleted, so we must search from the beginning. */

    for (pp_node = pp_head; p_node != *pp_node; ) {
      pp_node = &(*pp_node)->next;
      READ_MEMORY_BARRIER(); /* Stabilize *pp_node for next iteration. */
    }
  }

  *pp_node = p_node->next;
  return;
}

#endif

void __pthread_alt_lock(struct _pthread_fastlock * lock,
		        pthread_descr self)
{