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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-17 16:46:01 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-17 16:46:01 +0000
commit45c5dcc531fbc80d1e62e2e845723d423e5660b3 (patch)
treedadb594e059e1408573078215d0217718b2f6fcf /lib/talloc/doc/tutorial_threads.dox
parentInitial commit. (diff)
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Adding upstream version 0.16.1.upstream/0.16.1upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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+/**
+@page libtalloc_threads Chapter 8: Using threads with talloc
+
+@section Talloc and thread safety
+
+The talloc library is not internally thread-safe, in that accesses
+to variables on a talloc context are not controlled by mutexes or
+other thread-safe primitives.
+
+However, so long as talloc_disable_null_tracking() is called from
+the main thread to disable global variable access within talloc,
+then each thread can safely use its own top level talloc context
+allocated off the NULL context.
+
+For example:
+
+@code
+static void *thread_fn(void *arg)
+{
+ const char *ctx_name = (const char *)arg;
+ /*
+ * Create a new top level talloc hierarchy in
+ * this thread.
+ */
+ void *top_ctx = talloc_named_const(NULL, 0, "top");
+ if (top_ctx == NULL) {
+ return NULL;
+ }
+ sub_ctx = talloc_named_const(top_ctx, 100, ctx_name);
+ if (sub_ctx == NULL) {
+ return NULL;
+ }
+
+ /*
+ * Do more processing/talloc calls on top_ctx
+ * and its children.
+ */
+ ......
+
+ talloc_free(top_ctx);
+ return value;
+}
+@endcode
+
+is a perfectly safe use of talloc within a thread.
+
+The problem comes when one thread wishes to move some
+memory allocated on its local top level talloc context
+to another thread. Care must be taken to add data access
+exclusion to prevent memory corruption. One method would
+be to lock a mutex before any talloc call on each thread,
+but this would push the burden of total talloc thread-safety
+on the poor user of the library.
+
+A much easier way to transfer talloced memory between
+threads is by the use of an intermediate, mutex locked,
+intermediate variable.
+
+An example of this is below - taken from test code inside
+the talloc testsuite.
+
+The main thread creates 1000 sub-threads, and then accepts
+the transfer of some thread-talloc'ed memory onto its top
+level context from each thread in turn.
+
+A pthread mutex and condition variable are used to
+synchronize the transfer via the intermediate_ptr
+variable.
+
+@code
+/* Required sync variables. */
+static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t condvar = PTHREAD_COND_INITIALIZER;
+
+/* Intermediate talloc pointer for transfer. */
+static void *intermediate_ptr;
+
+/* Subthread. */
+static void *thread_fn(void *arg)
+{
+ int ret;
+ const char *ctx_name = (const char *)arg;
+ void *sub_ctx = NULL;
+ /*
+ * Do stuff that creates a new talloc hierarchy in
+ * this thread.
+ */
+ void *top_ctx = talloc_named_const(NULL, 0, "top");
+ if (top_ctx == NULL) {
+ return NULL;
+ }
+ sub_ctx = talloc_named_const(top_ctx, 100, ctx_name);
+ if (sub_ctx == NULL) {
+ return NULL;
+ }
+
+ /*
+ * Now transfer a pointer from our hierarchy
+ * onto the intermediate ptr.
+ */
+ ret = pthread_mutex_lock(&mtx);
+ if (ret != 0) {
+ talloc_free(top_ctx);
+ return NULL;
+ }
+
+ /* Wait for intermediate_ptr to be free. */
+ while (intermediate_ptr != NULL) {
+ ret = pthread_cond_wait(&condvar, &mtx);
+ if (ret != 0) {
+ talloc_free(top_ctx);
+ return NULL;
+ }
+ }
+
+ /* and move our memory onto it from our toplevel hierarchy. */
+ intermediate_ptr = talloc_move(NULL, &sub_ctx);
+
+ /* Tell the main thread it's ready for pickup. */
+ pthread_cond_broadcast(&condvar);
+ pthread_mutex_unlock(&mtx);
+
+ talloc_free(top_ctx);
+ return NULL;
+}
+
+/* Main thread. */
+
+#define NUM_THREADS 1000
+
+static bool test_pthread_talloc_passing(void)
+{
+ int i;
+ int ret;
+ char str_array[NUM_THREADS][20];
+ pthread_t thread_id;
+ void *mem_ctx;
+
+ /*
+ * Important ! Null tracking breaks threaded talloc.
+ * It *must* be turned off.
+ */
+ talloc_disable_null_tracking();
+
+ /* Main thread toplevel context. */
+ mem_ctx = talloc_named_const(NULL, 0, "toplevel");
+ if (mem_ctx == NULL) {
+ return false;
+ }
+
+ /*
+ * Spin off NUM_THREADS threads.
+ * They will use their own toplevel contexts.
+ */
+ for (i = 0; i < NUM_THREADS; i++) {
+ (void)snprintf(str_array[i],
+ 20,
+ "thread:%d",
+ i);
+ if (str_array[i] == NULL) {
+ return false;
+ }
+ ret = pthread_create(&thread_id,
+ NULL,
+ thread_fn,
+ str_array[i]);
+ if (ret != 0) {
+ return false;
+ }
+ }
+
+ /* Now wait for NUM_THREADS transfers of the talloc'ed memory. */
+ for (i = 0; i < NUM_THREADS; i++) {
+ ret = pthread_mutex_lock(&mtx);
+ if (ret != 0) {
+ talloc_free(mem_ctx);
+ return false;
+ }
+
+ /* Wait for intermediate_ptr to have our data. */
+ while (intermediate_ptr == NULL) {
+ ret = pthread_cond_wait(&condvar, &mtx);
+ if (ret != 0) {
+ talloc_free(mem_ctx);
+ return false;
+ }
+ }
+
+ /* and move it onto our toplevel hierarchy. */
+ (void)talloc_move(mem_ctx, &intermediate_ptr);
+
+ /* Tell the sub-threads we're ready for another. */
+ pthread_cond_broadcast(&condvar);
+ pthread_mutex_unlock(&mtx);
+ }
+
+ /* Dump the hierarchy. */
+ talloc_report(mem_ctx, stdout);
+ talloc_free(mem_ctx);
+ return true;
+}
+@endcode
+*/