specific.c

/*
 * Copyright (c) 2000 by Hewlett-Packard Company.  All rights reserved.
 *
 * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
 * OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
 *
 * Permission is hereby granted to use or copy this program
 * for any purpose, provided the above notices are retained on all copies.
 * Permission to modify the code and to distribute modified code is granted,
 * provided the above notices are retained, and a notice that the code was
 * modified is included with the above copyright notice.
 */

/* To determine type of tsd implementation; includes private/specific.h */
/* if needed.                                                           */
#include "private/thread_local_alloc.h"

#if defined(USE_CUSTOM_SPECIFIC)

/* A thread-specific data entry which will never appear valid to a      */
/* reader.  Used to fill in empty cache entries to avoid a check for 0. */
static const tse invalid_tse = { INVALID_QTID, 0, 0, INVALID_THREADID };

GC_INNER int
GC_key_create_inner(tsd **key_ptr)
{
  int i;
  int ret;
  tsd *result;

  GC_ASSERT(I_HOLD_LOCK());
  /* A quick alignment check, since we need atomic stores.    */
  GC_ASSERT(ADDR(&invalid_tse.next) % ALIGNMENT == 0);
  result = (tsd *)MALLOC_CLEAR(sizeof(tsd));
  if (NULL == result)
    return ENOMEM;
  ret = pthread_mutex_init(&result->lock, NULL);
  if (ret != 0)
    return ret;

  for (i = 0; i < TS_CACHE_SIZE; ++i) {
    result->cache[i] = (tse *)GC_CAST_AWAY_CONST_PVOID(&invalid_tse);
  }
#  ifdef GC_ASSERTIONS
  for (i = 0; i < TS_HASH_SIZE; ++i) {
    GC_ASSERT(NULL == result->hash[i]);
  }
#  endif
  *key_ptr = result;
  return 0;
}

/* Set the thread-local value associated with the key.  Should not  */
/* be used to overwrite a previously set value.                     */
GC_INNER int
GC_setspecific(tsd *key, void *value)
{
  pthread_t self = pthread_self();
  unsigned hash_val = TS_HASH(self);
  volatile tse *entry;

  GC_ASSERT(I_HOLD_LOCK());
  GC_ASSERT(self != INVALID_THREADID);
  /* Disable GC during malloc.        */
  GC_dont_gc++;
  entry = (volatile tse *)MALLOC_CLEAR(sizeof(tse));
  GC_dont_gc--;
  if (EXPECT(NULL == entry, FALSE))
    return ENOMEM;

  pthread_mutex_lock(&key->lock);
  entry->next = key->hash[hash_val];
#  ifdef GC_ASSERTIONS
  {
    tse *p;

    /* Ensure no existing entry.    */
    for (p = entry->next; p != NULL; p = p->next) {
      GC_ASSERT(!THREAD_EQUAL(p->thread, self));
    }
  }
#  endif
  entry->thread = self;
  entry->value = TS_HIDE_VALUE(value);
  GC_ASSERT(entry->qtid == INVALID_QTID);
  /* There can only be one writer at a time, but this needs to be     */
  /* atomic with respect to concurrent readers.                       */
  GC_cptr_store_release((volatile ptr_t *)&key->hash[hash_val],
                        (ptr_t)CAST_AWAY_VOLATILE_PVOID(entry));
  GC_dirty(CAST_AWAY_VOLATILE_PVOID(entry));
  GC_dirty(key->hash + hash_val);
  if (pthread_mutex_unlock(&key->lock) != 0)
    ABORT("pthread_mutex_unlock failed (setspecific)");
  return 0;
}

/* Remove thread-specific data for a given thread.  This function is    */
/* called at fork from the child process for all threads except for the */
/* survived one.  GC_remove_specific() should be called on thread exit. */
GC_INNER void
GC_remove_specific_after_fork(tsd *key, pthread_t t)
{
  unsigned hash_val = TS_HASH(t);
  tse *entry;
  tse *prev = NULL;

#  ifdef CAN_HANDLE_FORK
  /* Both GC_setspecific and GC_remove_specific should be called    */
  /* with the allocator lock held to ensure the consistency of      */
  /* the hash table in the forked child.                            */
  GC_ASSERT(I_HOLD_LOCK());
#  endif
  pthread_mutex_lock(&key->lock);
  for (entry = key->hash[hash_val];
       entry != NULL && !THREAD_EQUAL(entry->thread, t); entry = entry->next) {
    prev = entry;
  }
  /* Invalidate qtid field, since qtids may be reused, and a later    */
  /* cache lookup could otherwise find this entry.                    */
  if (entry != NULL) {
    entry->qtid = INVALID_QTID;
    if (NULL == prev) {
      key->hash[hash_val] = entry->next;
      GC_dirty(key->hash + hash_val);
    } else {
      prev->next = entry->next;
      GC_dirty(prev);
    }
    /* Atomic! Concurrent accesses still work.  They must, since      */
    /* readers do not lock.  We should not need a volatile access     */
    /* here, since both this and the preceding write should become    */
    /* visible no later than the pthread_mutex_unlock() call.         */
  }
  /* If we wanted to deallocate the entry, we'd first have to clear   */
  /* any cache entries pointing to it.  That probably requires        */
  /* additional synchronization, since we can't prevent a concurrent  */
  /* cache lookup, which should still be examining deallocated memory.*/
  /* This can only happen if the concurrent access is from another    */
  /* thread, and hence has missed the cache, but still...             */
#  ifdef LINT2
  GC_noop1_ptr(entry);
#  endif

  /* With GC, we're done, since the pointers from the cache will      */
  /* be overwritten, all local pointers to the entries will be        */
  /* dropped, and the entry will then be reclaimed.                   */
  if (pthread_mutex_unlock(&key->lock) != 0)
    ABORT("pthread_mutex_unlock failed (remove_specific after fork)");
}

/* Note that even the slow path doesn't lock.   */
GC_INNER void *
GC_slow_getspecific(tsd *key, size_t qtid, tse *volatile *cache_ptr)
{
  pthread_t self = pthread_self();
  tse *entry = key->hash[TS_HASH(self)];

  GC_ASSERT(qtid != INVALID_QTID);
  while (entry != NULL && !THREAD_EQUAL(entry->thread, self)) {
    entry = entry->next;
  }
  if (entry == NULL)
    return NULL;
  /* Set the cache entry.  It is safe to do this asynchronously.      */
  /* Either value is safe, though may produce spurious misses.        */
  /* We are replacing one qtid with another one for the same thread.  */
  AO_store(&entry->qtid, qtid);

  GC_cptr_store((volatile ptr_t *)cache_ptr, (ptr_t)entry);
  return TS_REVEAL_PTR(entry->value);
}

#  ifdef GC_ASSERTIONS
/* Check that that all elements of the data structure associated  */
/* with key are marked.                                           */
void
GC_check_tsd_marks(tsd *key)
{
  int i;
  tse *p;

  if (!GC_is_marked(GC_base(key))) {
    ABORT("Unmarked thread-specific-data table");
  }
  for (i = 0; i < TS_HASH_SIZE; ++i) {
    for (p = key->hash[i]; p != NULL; p = p->next) {
      if (!GC_is_marked(GC_base(p))) {
        ABORT_ARG1("Unmarked thread-specific-data entry", " at %p", (void *)p);
      }
    }
  }
  for (i = 0; i < TS_CACHE_SIZE; ++i) {
    p = key->cache[i];
    if (p != &invalid_tse && !GC_is_marked(GC_base(p))) {
      ABORT_ARG1("Unmarked cached thread-specific-data entry", " at %p",
                 (void *)p);
    }
  }
}
#  endif /* GC_ASSERTIONS */

#endif /* USE_CUSTOM_SPECIFIC */