libgc/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 #include "private/gc_priv.h" /* For GC_compare_and_exchange, GC_memory_barrier */
  15
  16 #if defined(GC_LINUX_THREADS) || defined(GC_NETBSD_THREADS)
  17
  18 #include "private/specific.h"
  19
  20 static tse invalid_tse = {INVALID_QTID, 0, 0, INVALID_THREADID};
  21                         /* A thread-specific data entry which will never        */
  22                         /* appear valid to a reader.  Used to fill in empty     */
  23                         /* cache entries to avoid a check for 0.                */
  24
  25 int PREFIXED(key_create) (tsd ** key_ptr, void (* destructor)(void *)) {
  26     int i;
  27     tsd * result = (tsd *)MALLOC_CLEAR(sizeof (tsd));
  28
  29     /* A quick alignment check, since we need atomic stores */
  30       GC_ASSERT((unsigned long)(&invalid_tse.next) % sizeof(tse *) == 0);
  31     if (0 == result) return ENOMEM;
  32     pthread_mutex_init(&(result -> lock), NULL);
  33     for (i = 0; i < TS_CACHE_SIZE; ++i) {
  34         result -> cache[i] = &invalid_tse;
  35     }
  36 #   ifdef GC_ASSERTIONS
  37       for (i = 0; i < TS_HASH_SIZE; ++i) {
  38         GC_ASSERT(result -> hash[i] == 0);
  39       }
  40 #   endif
  41     *key_ptr = result;
  42     return 0;
  43 }
  44
  45 int PREFIXED(setspecific) (tsd * key, void * value) {
  46     pthread_t self = pthread_self();
  47     int hash_val = HASH(self);
  48     volatile tse * entry = (volatile tse *)MALLOC_CLEAR(sizeof (tse));
  49
  50     GC_ASSERT(self != INVALID_THREADID);
  51     if (0 == entry) return ENOMEM;
  52     pthread_mutex_lock(&(key -> lock));
  53     /* Could easily check for an existing entry here.   */
  54     entry -> next = key -> hash[hash_val];
  55     entry -> thread = self;
  56     entry -> value = value;
  57     GC_ASSERT(entry -> qtid == INVALID_QTID);
  58     /* There can only be one writer at a time, but this needs to be     */
  59     /* atomic with respect to concurrent readers.                       */
  60     *(volatile tse **)(key -> hash + hash_val) = entry;
  61     pthread_mutex_unlock(&(key -> lock));
  62     return 0;
  63 }
  64
  65 /* Remove thread-specific data for this thread.  Should be called on    */
  66 /* thread exit.                                                         */
  67 void PREFIXED(remove_specific) (tsd * key) {
  68     pthread_t self = pthread_self();
  69     unsigned hash_val = HASH(self);
  70     tse *entry;
  71     tse **link = key -> hash + hash_val;
  72
  73     pthread_mutex_lock(&(key -> lock));
  74     entry = *link;
  75     while (entry != NULL && entry -> thread != self) {
  76         link = &(entry -> next);
  77         entry = *link;
  78     }
  79     /* Invalidate qtid field, since qtids may be reused, and a later    */
  80     /* cache lookup could otherwise find this entry.                    */
  81         entry -> qtid = INVALID_QTID;
  82     if (entry != NULL) {
  83         *link = entry -> next;
  84         /* Atomic! concurrent accesses still work.      */
  85         /* They must, since readers don't lock.         */
  86         /* We shouldn't need a volatile access here,    */
  87         /* since both this and the preceding write      */
  88         /* should become visible no later than          */
  89         /* the pthread_mutex_unlock() call.             */
  90     }
  91     /* If we wanted to deallocate the entry, we'd first have to clear   */
  92     /* any cache entries pointing to it.  That probably requires        */
  93     /* additional synchronization, since we can't prevent a concurrent  */
  94     /* cache lookup, which should still be examining deallocated memory.*/
  95     /* This can only happen if the concurrent access is from another    */
  96     /* thread, and hence has missed the cache, but still...             */
  97
  98     /* With GC, we're done, since the pointers from the cache will      */
  99     /* be overwritten, all local pointers to the entries will be        */
 100     /* dropped, and the entry will then be reclaimed.                   */
 101     pthread_mutex_unlock(&(key -> lock));
 102 }
 103
 104 /* Note that even the slow path doesn't lock.   */
 105 void *  PREFIXED(slow_getspecific) (tsd * key, unsigned long qtid,
 106                                     tse * volatile * cache_ptr) {
 107     pthread_t self = pthread_self();
 108     unsigned hash_val = HASH(self);
 109     tse *entry = key -> hash[hash_val];
 110
 111     GC_ASSERT(qtid != INVALID_QTID);
 112     while (entry != NULL && entry -> thread != self) {
 113         entry = entry -> next;
 114     }
 115     if (entry == NULL) return NULL;
 116     /* Set cache_entry.         */
 117         entry -> qtid = qtid;
 118                 /* It's safe to do this asynchronously.  Either value   */
 119                 /* is safe, though may produce spurious misses.         */
 120                 /* We're replacing one qtid with another one for the    */
 121                 /* same thread.                                         */
 122         *cache_ptr = entry;
 123                 /* Again this is safe since pointer assignments are     */
 124                 /* presumed atomic, and either pointer is valid.        */
 125     return entry -> value;
 126 }
 127
 128 #endif /* GC_LINUX_THREADS */