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