gc/specific.c

   1 /*
   2  * Copyright (c) 2000 by Hewlett-Packard Company.  All rights reserved.
   3  *
   4  * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
   5  * OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
   6  *
   7  * Permission is hereby granted to use or copy this program
   8  * for any purpose,  provided the above notices are retained on all copies.
   9  * Permission to modify the code and to distribute modified code is granted,
  10  * provided the above notices are retained, and a notice that the code was
  11  * modified is included with the above copyright notice.
  12  */
  13
  14 #if defined(GC_LINUX_THREADS)
  15
  16 #include "private/gc_priv.h" /* For GC_compare_and_exchange, GC_memory_barrier */
  17 #include "private/specific.h"
  18
  19 static tse invalid_tse;         /* 0 qtid is guaranteed to be invalid   */
  20
  21 int PREFIXED(key_create) (tsd ** key_ptr, void (* destructor)(void *)) {
  22     int i;
  23     tsd * result = (tsd *)MALLOC_CLEAR(sizeof (tsd));
  24
  25     if (0 == result) return ENOMEM;
  26     pthread_mutex_init(&(result -> lock), NULL);
  27     for (i = 0; i < TS_CACHE_SIZE; ++i) {
  28         result -> cache[i] = &invalid_tse;
  29     }
  30     *key_ptr = result;
  31     return 0;
  32 }
  33
  34 int PREFIXED(setspecific) (tsd * key, void * value) {
  35     pthread_t self = pthread_self();
  36     int hash_val = HASH(self);
  37     volatile tse * entry = (volatile tse *)MALLOC_CLEAR(sizeof (tse));
  38
  39     if (0 == entry) return ENOMEM;
  40     pthread_mutex_lock(&(key -> lock));
  41     /* Could easily check for an existing entry here.   */
  42     entry -> next = key -> hash[hash_val];
  43     entry -> thread = self;
  44     entry -> value = value;
  45     /* There can only be one writer at a time, but this needs to be     */
  46     /* atomic with respect to concurrent readers.                       */
  47     *(volatile tse **)(key -> hash + hash_val) = entry;
  48     pthread_mutex_unlock(&(key -> lock));
  49     return 0;
  50 }
  51
  52 /* Remove thread-specific data for this thread.  Should be called on    */
  53 /* thread exit.                                                         */
  54 void PREFIXED(remove_specific) (tsd * key) {
  55     pthread_t self = pthread_self();
  56     unsigned hash_val = HASH(self);
  57     tse *entry;
  58     tse **link = key -> hash + hash_val;
  59
  60     pthread_mutex_lock(&(key -> lock));
  61     entry = *link;
  62     while (entry != NULL && entry -> thread != self) {
  63         link = &(entry -> next);
  64         entry = *link;
  65     }
  66     /* Invalidate qtid field, since qtids may be reused, and a later    */
  67     /* cache lookup could otherwise find this entry.                    */
  68         entry -> qtid = INVALID_QTID;
  69     if (entry != NULL) {
  70         *link = entry -> next;
  71         /* Atomic! concurrent accesses still work.      */
  72         /* They must, since readers don't lock.         */
  73     }
  74     /* If we wanted to deallocate the entry, we'd first have to clear   */
  75     /* any cache entries pointing to it.  That probably requires        */
  76     /* additional synchronization, since we can't prevent a concurrent  */
  77     /* cache lookup, which should still be examining deallocated memory.*/
  78     /* This can only happen if the concurrent access is from another    */
  79     /* thread, and hence has missed the cache, but still...             */
  80
  81     /* With GC, we're done, since the pointers from the cache will      */
  82     /* be overwritten, all local pointers to the entries will be        */
  83     /* dropped, and the entry will then be reclaimed.                   */
  84     pthread_mutex_unlock(&(key -> lock));
  85 }
  86
  87 /* Note that even the slow path doesn't lock.   */
  88 void *  PREFIXED(slow_getspecific) (tsd * key, unsigned long qtid,
  89                                     tse * volatile * cache_ptr) {
  90     pthread_t self = pthread_self();
  91     unsigned hash_val = HASH(self);
  92     tse *entry = key -> hash[hash_val];
  93
  94     while (entry != NULL && entry -> thread != self) {
  95         entry = entry -> next;
  96     }
  97     if (entry == NULL) return NULL;
  98     /* Set cache_entry.         */
  99         entry -> qtid = qtid;
 100                 /* It's safe to do this asynchronously.  Either value   */
 101                 /* is safe, though may produce spurious misses.         */
 102         *cache_ptr = entry;
 103                 /* Again this is safe since pointer assignments are     */
 104                 /* presumed atomic, and either pointer is valid.        */
 105     return entry -> value;
 106 }
 107
 108 #endif /* GC_LINUX_THREADS */