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atomic_ops_stack.c

/*  
 * Copyright (c) 2005 Hewlett-Packard Development Company, L.P.
 * Original Author: Hans Boehm
 *
 * This file may be redistributed and/or modified under the
 * terms of the GNU General Public License as published by the Free Software
 * Foundation; either version 2, or (at your option) any later version.
 * 
 * It 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 General Public License in the
 * file doc/COPYING for more details.
 */

#if defined(HAVE_CONFIG_H)
# include "config.h"
#endif

#include <string.h>
#include <stdlib.h>
#include <assert.h>
#define AO_REQUIRE_CAS
#include "atomic_ops_stack.h"

#if defined(_MSC_VER) \
    || defined(_WIN32) && !defined(__CYGWIN32__) && !defined(__CYGWIN__)
  /* AO_pause not defined elsewhere */
  /* FIXME: At least AO_spin should be factored out.  */
#include <windows.h>

AO_t dummy;

/* Spin for 2**n units. */
static void AO_spin(int n)
{
  int i;
  AO_T j = AO_load(&dummy);

  for (i = 0; i < (2 << n); ++i)
    {
       j *= 5;
       j -= 4;
    }
  AO_store(&dummy, j);
}

void AO_pause(int n)
{
    if (n < 12)
      AO_spin(n);
    else
      {
        DWORD msecs;

      /* Short async-signal-safe sleep. */
      msecs = (n > 18? 100 : (1 << (n - 12)));
      Sleep(msecs);
      }
}

#else 

/* AO_pause is available elsewhere */

extern void AO_pause(int);

#endif

#ifdef AO_USE_ALMOST_LOCK_FREE

/* LIFO linked lists based on compare-and-swap.  We need to avoid */
/* the case of a node deleton and reinsertion while I'm deleting  */
/* it, since that may cause my CAS to succeed eventhough the next */
/* pointer is now wrong.  Our solution is not fully lock-free, but it   */
/* is good enough for signal handlers, provided we have a suitably low  */
/* bound on the number of recursive signal handler reentries.           */
/* A list consists of a first pointer and a blacklist             */
/* of pointer values that are currently being removed.  No list element */
/* on the blacklist may be inserted.  If we would otherwise do so, we   */
/* are allowed to insert a variant that differs only in the least */
/* significant, ignored, bits.  If the list is full, we wait.           */

/* Crucial observation: A particular padded pointer x (i.e. pointer     */
/* plus arbitrary low order bits) can never be newly inserted into      */
/* a list while it's in the corresponding auxiliary data structure.     */

/* The second argument is a pointer to the link field of the element    */
/* to be inserted.                                          */
/* Both list headers and link fields contain "perturbed" pointers, i.e. */
/* pointers with extra bits "or"ed into the low order bits.       */
void
AO_stack_push_explicit_aux_release(volatile AO_t *list, AO_t *x,
                           AO_stack_aux *a)
{
  int i;
  AO_t x_bits = (AO_t)x;
  AO_t next;
  
  /* No deletions of x can start here, since x is not currently in the  */
  /* list.                                            */
 retry:
# if AO_BL_SIZE == 2
  {
    /* Start all loads as close to concurrently as possible. */
    AO_t entry1 = AO_load(a -> AO_stack_bl);
    AO_t entry2 = AO_load(a -> AO_stack_bl + 1);
    if (entry1 == x_bits || entry2 == x_bits)
      {
      /* Entry is currently being removed.  Change it a little.     */
        ++x_bits;
        if ((x_bits & AO_BIT_MASK) == 0)
          /* Version count overflowed;         */
          /* EXTREMELY unlikely, but possible. */
          x_bits = (AO_t)x;
      goto retry;
      }
  }
# else
    for (i = 0; i < AO_BL_SIZE; ++i)
      {
        if (AO_load(a -> AO_stack_bl + i) == x_bits)
          {
          /* Entry is currently being removed.  Change it a little.     */
            ++x_bits;
            if ((x_bits & AO_BIT_MASK) == 0)
              /* Version count overflowed;         */
            /* EXTREMELY unlikely, but possible. */
              x_bits = (AO_t)x;
          goto retry;
          }
      }
# endif
  /* x_bits is not currently being deleted */
  do
    {
      next = AO_load(list);
      *x = next;
    }
  while(!AO_compare_and_swap_release(list, next, x_bits));
}

/*
 * I concluded experimentally that checking a value first before
 * performing a compare-and-swap is usually beneficial on X86, but
 * slows things down appreciably with contention on Itanium.
 * ince the Itanium behavior makes more sense to me (more cache line
 * movement unless we're mostly reading, but back-off should guard
 * against that), we take Itanium as the default.  Measurements on
 * other multiprocessor architectures would be useful.  (On a uniprocessor,
 * the initial check is almost certainly a very small loss.) - HB
 */
#ifdef __i386__
# define PRECHECK(a) (a) == 0 &&
#else
# define PRECHECK(a)
#endif

AO_t *
AO_stack_pop_explicit_aux_acquire(volatile AO_t *list, AO_stack_aux * a)
{
  unsigned i;
  int j = 0;
  AO_t first;
  AO_t * first_ptr;
  AO_t next;

 retry:
  first = AO_load(list);
  if (0 == first) return 0;
  /* Insert first into aux black list.                            */
  /* This may spin if more than AO_BL_SIZE removals using auxiliary     */
  /* structure a are currently in progress.                       */
  for (i = 0; ; )
    {
      if (PRECHECK(a -> AO_stack_bl[i])
        AO_compare_and_swap_acquire(a->AO_stack_bl+i, 0, first))
        break;
      ++i;
      if ( i >= AO_BL_SIZE )
      {
        i = 0;
        AO_pause(++j);
      }
    }
  assert(i >= 0 && i < AO_BL_SIZE);
  assert(a -> AO_stack_bl[i] == first);
  /* First is on the auxiliary black list.  It may be removed by  */
  /* another thread before we get to it, but a new insertion of x */
  /* cannot be started here.                                */
  /* Only we can remove it from the black list.                   */
  /* We need to make sure that first is still the first entry on the    */
  /* list.  Otherwise it's possible that a reinsertion of it was  */
  /* already started before we added the black list entry.        */
  if (first != AO_load(list)) {
    AO_store_release(a->AO_stack_bl+i, 0);
    goto retry;
  }
  first_ptr = AO_REAL_NEXT_PTR(first);
  next = AO_load(first_ptr);
  if (!AO_compare_and_swap_release(list, first, next)) {
    AO_store_release(a->AO_stack_bl+i, 0);
    goto retry;
  }
  assert(*list != first);
  /* Since we never insert an entry on the black list, this cannot have */
  /* succeeded unless first remained on the list while we were running. */
  /* Thus its next link cannot have changed out from under us, and we   */
  /* removed exactly one entry and preserved the rest of the list.      */
  /* Note that it is quite possible that an additional entry was  */
  /* inserted and removed while we were running; this is OK since the   */
  /* part of the list following first must have remained unchanged, and */
  /* first must again have been at the head of the list when the  */
  /* compare_and_swap succeeded.                            */
  AO_store_release(a->AO_stack_bl+i, 0);
  return first_ptr;
}

#else /* ! USE_ALMOST_LOCK_FREE */

/* Better names for fields in AO_stack_t */
#define ptr AO_val2
#define version AO_val1

#if defined(AO_HAVE_compare_double_and_swap_double)

void AO_stack_push_release(AO_stack_t *list, AO_t *element)
{
    AO_t next;

    do {
      next = AO_load(&(list -> ptr));
      *element = next;
    } while (!AO_compare_and_swap_release
                ( &(list -> ptr), next, (AO_t) element));
    /* This uses a narrow CAS here, an old optimization suggested */
    /* by Treiber.  Pop is still safe, since we run into the ABA  */
    /* problem only if there were both interveining "pop"s and "push"es.*/
    /* Inthat case we still see a change inthe version number.          */
}

AO_t *AO_stack_pop_acquire(AO_stack_t *list)
{
    AO_t *cptr;
    AO_t next;
    AO_t cversion;

    do {
      /* Version must be loaded first.    */
      cversion = AO_load_acquire(&(list -> version));
      cptr = (AO_t *)AO_load(&(list -> ptr));
      if (cptr == 0) return 0;
      next = *cptr;
    } while (!AO_compare_double_and_swap_double_release
                (list, cversion, (AO_t) cptr, cversion+1, (AO_t) next));
    return cptr;
}


#elif defined(AO_HAVE_compare_and_swap_double)

/* Needed for future IA64 processors.  No current clients? */

#error Untested!  Probably doesnt work.

/* We have a wide CAS, but only does an AO_t-wide comparison.     */
/* We can't use the Treiber optimization, since we only check     */
/* for an unchanged version number, not an unchanged pointer.     */
void AO_stack_push_release(AO_stack_t *list, AO_t *element)
{
    AO_t version;
    AO_t next_ptr;

    do {
      /* Again version must be loaded first, for different reason.      */
      version = AO_load_acquire(&(list -> version));
      next_ptr = AO_load(&(list -> ptr));
      *element = next_ptr;
    } while (!AO_compare_and_swap_double_release(
                     list, version,
                     version+1, (AO_t) element));
}

AO_t *AO_stack_pop_acquire(AO_stack_t *list)
{
    AO_t *cptr;
    AO_t next;
    AO_t cversion;

    do {
      cversion = AO_load_acquire(&(list -> version));
      cptr = (AO_t *)AO_load(&(list -> ptr));
      if (cptr == 0) return 0;
      next = *cptr;
    } while (!AO_compare_double_and_swap_double_release
                (list, cversion, (AO_t) cptr, cversion+1, next));
    return cptr;
}


#endif /* AO_HAVE_compare_and_swap_double */

#endif /* ! USE_ALMOST_LOCK_FREE */

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