X-Git-Url: https://git.lttng.org/?p=urcu.git;a=blobdiff_plain;f=urcu-qsbr.c;h=56d86f9e561d6ea8fc372eed83ffc36f35dc358b;hp=ccecab4e7b02b63d80f6c87085486dee0a5452f5;hb=64c4cafd597aff7423be003dcf41735904126f66;hpb=727f819de5737f4be5b9f7e50aacf84c7a3f7999

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index ccecab4..56d86f9 100644
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -37,34 +37,29 @@
 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
 #include "urcu-qsbr.h"
 
-pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
+void __attribute__((destructor)) rcu_exit(void);
+
+static pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+int gp_futex;
 
 /*
  * Global grace period counter.
  */
-long urcu_gp_ctr = 0;
+unsigned long urcu_gp_ctr = RCU_GP_ONLINE;
 
 /*
  * Written to only by each individual reader. Read by both the reader and the
  * writers.
  */
-long __thread rcu_reader_qs_gp;
-
-/* Thread IDs of registered readers */
-#define INIT_NUM_THREADS 4
-
-struct reader_registry {
-	pthread_t tid;
-	long *rcu_reader_qs_gp;
-};
+struct urcu_reader __thread urcu_reader;
 
 #ifdef DEBUG_YIELD
 unsigned int yield_active;
 unsigned int __thread rand_yield;
 #endif
 
-static struct reader_registry *registry;
-static int num_readers, alloc_readers;
+static LIST_HEAD(registry);
 
 static void internal_urcu_lock(void)
 {
@@ -99,75 +94,175 @@ static void internal_urcu_unlock(void)
 	}
 }
 
-#ifdef HAS_INCOHERENT_CACHES
-static void force_mb_single_thread(struct reader_registry *index)
-{
-	smp_mb();
-}
-#endif /* #ifdef HAS_INCOHERENT_CACHES */
-
-static void force_mb_all_threads(void)
+/*
+ * synchronize_rcu() waiting. Single thread.
+ */
+static void wait_gp(void)
 {
-	smp_mb();
+	/* Read reader_gp before read futex */
+	smp_rmb();
+	if (uatomic_read(&gp_futex) == -1)
+		futex(&gp_futex, FUTEX_WAIT, -1,
+		      NULL, NULL, 0);
 }
 
 static void wait_for_quiescent_state(void)
 {
-	struct reader_registry *index;
+	LIST_HEAD(qsreaders);
+	int wait_loops = 0;
+	struct urcu_reader *index, *tmp;
 
-	if (!registry)
+	if (list_empty(&registry))
 		return;
 	/*
 	 * Wait for each thread rcu_reader_qs_gp count to become 0.
 	 */
-	for (index = registry; index < registry + num_readers; index++) {
-#ifndef HAS_INCOHERENT_CACHES
-		while (rcu_gp_ongoing(index->rcu_reader_qs_gp) &&
-		       (*index->rcu_reader_qs_gp - urcu_gp_ctr < 0))
-			cpu_relax();
-#else /* #ifndef HAS_INCOHERENT_CACHES */
-		int wait_loops = 0;
-		/*
-		 * BUSY-LOOP. Force the reader thread to commit its
-		 * rcu_reader_qs_gp update to memory if we wait for too long.
-		 */
-		while (rcu_gp_ongoing(index->rcu_reader_qs_gp) &&
-		       (*index->rcu_reader_qs_gp - urcu_gp_ctr < 0)) {
-			if (wait_loops++ == KICK_READER_LOOPS) {
-				force_mb_single_thread(index);
-				wait_loops = 0;
+	for (;;) {
+		wait_loops++;
+		if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
+			uatomic_dec(&gp_futex);
+			/* Write futex before read reader_gp */
+			smp_mb();
+		}
+
+		list_for_each_entry_safe(index, tmp, &registry, head) {
+			if (!rcu_gp_ongoing(&index->ctr))
+				list_move(&index->head, &qsreaders);
+		}
+
+		if (list_empty(&registry)) {
+			if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
+				/* Read reader_gp before write futex */
+				smp_mb();
+				uatomic_set(&gp_futex, 0);
+			}
+			break;
+		} else {
+			if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
+				wait_gp();
 			} else {
+#ifndef HAS_INCOHERENT_CACHES
 				cpu_relax();
+#else /* #ifndef HAS_INCOHERENT_CACHES */
+				smp_mb();
+#endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 			}
 		}
-#endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 	}
+	/* put back the reader list in the registry */
+	list_splice(&qsreaders, &registry);
 }
 
+/*
+ * Using a two-subphases algorithm for architectures with smaller than 64-bit
+ * long-size to ensure we do not encounter an overflow bug.
+ */
+
+#if (BITS_PER_LONG < 64)
+/*
+ * called with urcu_mutex held.
+ */
+static void switch_next_urcu_qparity(void)
+{
+	STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_CTR);
+}
+
+void synchronize_rcu(void)
+{
+	unsigned long was_online;
+
+	was_online = urcu_reader.ctr;
+
+	/* All threads should read qparity before accessing data structure
+	 * where new ptr points to.
+	 */
+	/* Write new ptr before changing the qparity */
+	smp_mb();
+
+	/*
+	 * Mark the writer thread offline to make sure we don't wait for
+	 * our own quiescent state. This allows using synchronize_rcu() in
+	 * threads registered as readers.
+	 */
+	if (was_online)
+		STORE_SHARED(urcu_reader.ctr, 0);
+
+	internal_urcu_lock();
+
+	switch_next_urcu_qparity();	/* 0 -> 1 */
+
+	/*
+	 * Must commit qparity update to memory before waiting for parity
+	 * 0 quiescent state. Failure to do so could result in the writer
+	 * waiting forever while new readers are always accessing data (no
+	 * progress).
+	 * Ensured by STORE_SHARED and LOAD_SHARED.
+	 */
+
+	/*
+	 * Wait for previous parity to be empty of readers.
+	 */
+	wait_for_quiescent_state();	/* Wait readers in parity 0 */
+
+	/*
+	 * Must finish waiting for quiescent state for parity 0 before
+	 * committing qparity update to memory. Failure to do so could result in
+	 * the writer waiting forever while new readers are always accessing
+	 * data (no progress).
+	 * Ensured by STORE_SHARED and LOAD_SHARED.
+	 */
+
+	switch_next_urcu_qparity();	/* 1 -> 0 */
+
+	/*
+	 * Must commit qparity update to memory before waiting for parity
+	 * 1 quiescent state. Failure to do so could result in the writer
+	 * waiting forever while new readers are always accessing data (no
+	 * progress).
+	 * Ensured by STORE_SHARED and LOAD_SHARED.
+	 */
+
+	/*
+	 * Wait for previous parity to be empty of readers.
+	 */
+	wait_for_quiescent_state();	/* Wait readers in parity 1 */
+
+	internal_urcu_unlock();
+
+	/*
+	 * Finish waiting for reader threads before letting the old ptr being
+	 * freed.
+	 */
+	if (was_online)
+		_STORE_SHARED(urcu_reader.ctr, LOAD_SHARED(urcu_gp_ctr));
+	smp_mb();
+}
+#else /* !(BITS_PER_LONG < 64) */
 void synchronize_rcu(void)
 {
-	int was_online;
+	unsigned long was_online;
 
-	was_online = rcu_reader_qs_gp & 1;
+	was_online = urcu_reader.ctr;
 
 	/*
 	 * Mark the writer thread offline to make sure we don't wait for
 	 * our own quiescent state. This allows using synchronize_rcu() in
 	 * threads registered as readers.
 	 */
+	smp_mb();
 	if (was_online)
-		_rcu_thread_offline();
+		STORE_SHARED(urcu_reader.ctr, 0);
 
 	internal_urcu_lock();
-	force_mb_all_threads();
-	urcu_gp_ctr += 2;
+	STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr + RCU_GP_CTR);
 	wait_for_quiescent_state();
-	force_mb_all_threads();
 	internal_urcu_unlock();
 
 	if (was_online)
-		_rcu_thread_online();
+		_STORE_SHARED(urcu_reader.ctr, LOAD_SHARED(urcu_gp_ctr));
+	smp_mb();
 }
+#endif  /* !(BITS_PER_LONG < 64) */
 
 /*
  * library wrappers to be used by non-LGPL compatible source code.
@@ -183,32 +278,6 @@ void rcu_read_unlock(void)
 	_rcu_read_unlock();
 }
 
-void *rcu_dereference(void *p)
-{
-	return _rcu_dereference(p);
-}
-
-void *rcu_assign_pointer_sym(void **p, void *v)
-{
-	wmb();
-	return STORE_SHARED(p, v);
-}
-
-void *rcu_xchg_pointer_sym(void **p, void *v)
-{
-	wmb();
-	return xchg(p, v);
-}
-
-void *rcu_publish_content_sym(void **p, void *v)
-{
-	void *oldptr;
-
-	oldptr = _rcu_xchg_pointer(p, v);
-	synchronize_rcu();
-	return oldptr;
-}
-
 void rcu_quiescent_state(void)
 {
 	_rcu_quiescent_state();
@@ -224,58 +293,13 @@ void rcu_thread_online(void)
 	_rcu_thread_online();
 }
 
-static void rcu_add_reader(pthread_t id)
-{
-	struct reader_registry *oldarray;
-
-	if (!registry) {
-		alloc_readers = INIT_NUM_THREADS;
-		num_readers = 0;
-		registry =
-			malloc(sizeof(struct reader_registry) * alloc_readers);
-	}
-	if (alloc_readers < num_readers + 1) {
-		oldarray = registry;
-		registry = malloc(sizeof(struct reader_registry)
-				* (alloc_readers << 1));
-		memcpy(registry, oldarray,
-			sizeof(struct reader_registry) * alloc_readers);
-		alloc_readers <<= 1;
-		free(oldarray);
-	}
-	registry[num_readers].tid = id;
-	/* reference to the TLS of _this_ reader thread. */
-	registry[num_readers].rcu_reader_qs_gp = &rcu_reader_qs_gp;
-	num_readers++;
-}
-
-/*
- * Never shrink (implementation limitation).
- * This is O(nb threads). Eventually use a hash table.
- */
-static void rcu_remove_reader(pthread_t id)
-{
-	struct reader_registry *index;
-
-	assert(registry != NULL);
-	for (index = registry; index < registry + num_readers; index++) {
-		if (pthread_equal(index->tid, id)) {
-			memcpy(index, &registry[num_readers - 1],
-				sizeof(struct reader_registry));
-			registry[num_readers - 1].tid = 0;
-			registry[num_readers - 1].rcu_reader_qs_gp = NULL;
-			num_readers--;
-			return;
-		}
-	}
-	/* Hrm not found, forgot to register ? */
-	assert(0);
-}
-
 void rcu_register_thread(void)
 {
+	urcu_reader.tid = pthread_self();
+	assert(urcu_reader.ctr == 0);
+
 	internal_urcu_lock();
-	rcu_add_reader(pthread_self());
+	list_add(&urcu_reader.head, &registry);
 	internal_urcu_unlock();
 	_rcu_thread_online();
 }
@@ -288,6 +312,11 @@ void rcu_unregister_thread(void)
 	 */
 	_rcu_thread_offline();
 	internal_urcu_lock();
-	rcu_remove_reader(pthread_self());
+	list_del(&urcu_reader.head);
 	internal_urcu_unlock();
 }
+
+void rcu_exit(void)
+{
+	assert(list_empty(&registry));
+}