X-Git-Url: http://git.liburcu.org/?p=urcu.git;a=blobdiff_plain;f=urcu.c;h=475bebf526dfd1b9cc8cf78b87d8804ded7a1b6f;hp=9fde6238f695b06db846866a704f9ed595bd6d0c;hb=2d6debff95ad695255d2ea9d590d1e418590b238;hpb=f4a486ac095bd844d0c18995737d589b8f085b69

diff --git a/urcu.c b/urcu.c
index 9fde623..475bebf 100644
--- a/urcu.c
+++ b/urcu.c
@@ -19,17 +19,26 @@
 
 pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
 
-/* Global grace period counter */
-int urcu_gp_ctr;
+/*
+ * Global grace period counter.
+ * Contains the current RCU_GP_CTR_BIT.
+ * Also has a RCU_GP_CTR_BIT of 1, to accelerate the reader fast path.
+ * Written to only by writer with mutex taken. Read by both writer and readers.
+ */
+long urcu_gp_ctr = RCU_GP_COUNT;
 
-int __thread urcu_active_readers;
+/*
+ * Written to only by each individual reader. Read by both the reader and the
+ * writers.
+ */
+long __thread urcu_active_readers;
 
 /* Thread IDs of registered readers */
 #define INIT_NUM_THREADS 4
 
 struct reader_data {
 	pthread_t tid;
-	int *urcu_active_readers;
+	long *urcu_active_readers;
 };
 
 #ifdef DEBUG_YIELD
@@ -39,16 +48,23 @@ unsigned int __thread rand_yield;
 
 static struct reader_data *reader_data;
 static int num_readers, alloc_readers;
+#ifndef DEBUG_FULL_MB
 static int sig_done;
+#endif
 
 void internal_urcu_lock(void)
 {
+#if 0
 	int ret;
+	/* Mutex sleeping does not play well with busy-waiting loop. */
 	ret = pthread_mutex_lock(&urcu_mutex);
 	if (ret) {
 		perror("Error in pthread mutex lock");
 		exit(-1);
 	}
+#endif
+	while (pthread_mutex_trylock(&urcu_mutex) != 0)
+		cpu_relax();
 }
 
 void internal_urcu_unlock(void)
@@ -67,37 +83,72 @@ void internal_urcu_unlock(void)
  */
 static void switch_next_urcu_qparity(void)
 {
-	urcu_gp_ctr ^= RCU_GP_CTR_BIT;
+	STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_CTR_BIT);
+}
+
+#ifdef DEBUG_FULL_MB
+static void force_mb_single_thread(pthread_t tid)
+{
+	smp_mb();
+}
+
+static void force_mb_all_threads(void)
+{
+	smp_mb();
+}
+#else
+
+static void force_mb_single_thread(pthread_t tid)
+{
+	assert(reader_data);
+	sig_done = 0;
+	/*
+	 * pthread_kill has a smp_mb(). But beware, we assume it performs
+	 * a cache flush on architectures with non-coherent cache. Let's play
+	 * safe and don't assume anything : we use smp_mc() to make sure the
+	 * cache flush is enforced.
+	 * smp_mb();    write sig_done before sending the signals
+	 */
+	smp_mc();	/* write sig_done before sending the signals */
+	pthread_kill(tid, SIGURCU);
+	/*
+	 * Wait for sighandler (and thus mb()) to execute on every thread.
+	 * BUSY-LOOP.
+	 */
+	while (LOAD_SHARED(sig_done) < 1)
+		cpu_relax();
+	smp_mb();	/* read sig_done before ending the barrier */
 }
 
 static void force_mb_all_threads(void)
 {
 	struct reader_data *index;
 	/*
-	 * Ask for each threads to execute a mb() so we can consider the
+	 * Ask for each threads to execute a smp_mb() so we can consider the
 	 * compiler barriers around rcu read lock as real memory barriers.
 	 */
 	if (!reader_data)
 		return;
-	debug_yield_write();
 	sig_done = 0;
-	debug_yield_write();
-	mb();	/* write sig_done before sending the signals */
-	debug_yield_write();
-	for (index = reader_data; index < reader_data + num_readers; index++) {
+	/*
+	 * pthread_kill has a smp_mb(). But beware, we assume it performs
+	 * a cache flush on architectures with non-coherent cache. Let's play
+	 * safe and don't assume anything : we use smp_mc() to make sure the
+	 * cache flush is enforced.
+	 * smp_mb();    write sig_done before sending the signals
+	 */
+	smp_mc();	/* write sig_done before sending the signals */
+	for (index = reader_data; index < reader_data + num_readers; index++)
 		pthread_kill(index->tid, SIGURCU);
-		debug_yield_write();
-	}
 	/*
 	 * Wait for sighandler (and thus mb()) to execute on every thread.
 	 * BUSY-LOOP.
 	 */
-	while (sig_done < num_readers)
-		barrier();
-	debug_yield_write();
-	mb();	/* read sig_done before ending the barrier */
-	debug_yield_write();
+	while (LOAD_SHARED(sig_done) < num_readers)
+		cpu_relax();
+	smp_mb();	/* read sig_done before ending the barrier */
 }
+#endif
 
 void wait_for_quiescent_state(void)
 {
@@ -105,50 +156,80 @@ void wait_for_quiescent_state(void)
 
 	if (!reader_data)
 		return;
-	/* Wait for each thread urcu_active_readers count to become 0.
+	/*
+	 * Wait for each thread urcu_active_readers count to become 0.
 	 */
 	for (index = reader_data; index < reader_data + num_readers; index++) {
+		int wait_loops = 0;
 		/*
-		 * BUSY-LOOP.
+		 * BUSY-LOOP. Force the reader thread to commit its
+		 * urcu_active_readers update to memory if we wait for too long.
 		 */
-		while (rcu_old_gp_ongoing(index->urcu_active_readers))
-			barrier();
+		while (rcu_old_gp_ongoing(index->urcu_active_readers)) {
+			if (wait_loops++ == KICK_READER_LOOPS) {
+				force_mb_single_thread(index->tid);
+				wait_loops = 0;
+			} else {
+				cpu_relax();
+			}
+		}
 	}
-	/*
-	 * Locally : read *index->urcu_active_readers before freeing old
-	 * pointer.
-	 * Remote (reader threads) : Order urcu_qparity update and other
-	 * thread's quiescent state counter read.
-	 */
-	force_mb_all_threads();
 }
 
-static void switch_qparity(void)
+void synchronize_rcu(void)
 {
-	/* All threads should read qparity before accessing data structure. */
-	/* Write ptr before changing the qparity */
+	internal_urcu_lock();
+
+	/* All threads should read qparity before accessing data structure
+	 * where new ptr points to. Must be done within internal_urcu_lock
+	 * because it iterates on reader threads.*/
+	/* Write new ptr before changing the qparity */
 	force_mb_all_threads();
-	debug_yield_write();
-	switch_next_urcu_qparity();
-	debug_yield_write();
+
+	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_for_quiescent_state();	/* Wait readers in parity 0 */
 
-void synchronize_rcu(void)
-{
-	debug_yield_write();
-	internal_urcu_lock();
-	debug_yield_write();
-	switch_qparity();
-	debug_yield_write();
-	switch_qparity();
-	debug_yield_write();
-	internal_urcu_lock();
-	debug_yield_write();
+	/*
+	 * 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 */
+
+	/* Finish waiting for reader threads before letting the old ptr being
+	 * freed. Must be done within internal_urcu_lock because it iterates on
+	 * reader threads. */
+	force_mb_all_threads();
+
+	internal_urcu_unlock();
 }
 
 void urcu_add_reader(pthread_t id)
@@ -213,9 +294,15 @@ void urcu_unregister_thread(void)
 	internal_urcu_unlock();
 }
 
+#ifndef DEBUG_FULL_MB
 void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
 {
-	mb();
+	/*
+	 * Executing this smp_mb() is the only purpose of this signal handler.
+	 * It punctually promotes barrier() into smp_mb() on every thread it is
+	 * executed on.
+	 */
+	smp_mb();
 	atomic_inc(&sig_done);
 }
 
@@ -245,3 +332,4 @@ void __attribute__((destructor)) urcu_exit(void)
 	assert(act.sa_sigaction == sigurcu_handler);
 	free(reader_data);
 }
+#endif