X-Git-Url: https://git.lttng.org/?p=urcu.git;a=blobdiff_plain;f=urcu-qsbr.c;h=bb98a199d5138664c4f2e03b14ca7a45cde93c98;hp=fd0ebf6c4a710ae2bed98e6a919f424eb05bc992;hb=efafc824ca148c6b2ca7f95dc86d2aaa38d4a923;hpb=9f1621ca8d6ce9178343ae24aeef406c0ac2564a

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index fd0ebf6..bb98a19 100644
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -1,7 +1,7 @@
 /*
- * urcu.c
+ * urcu-qsbr.c
  *
- * Userspace RCU library
+ * Userspace RCU QSBR library
  *
  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
  * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
@@ -32,47 +32,36 @@
 #include <errno.h>
 #include <poll.h>
 
-#include "urcu-static.h"
+#define BUILD_QSBR_LIB
+#include "urcu-qsbr-static.h"
 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
-#include "urcu.h"
+#include "urcu-qsbr.h"
 
-void __attribute__((constructor)) urcu_init(void);
-void __attribute__((destructor)) urcu_exit(void);
+void __attribute__((destructor)) rcu_exit(void);
 
-int init_done;
+static pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
 
-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 urcu_active_readers;
-
-/* Thread IDs of registered readers */
-#define INIT_NUM_THREADS 4
-
-struct reader_registry {
-	pthread_t tid;
-	long *urcu_active_readers;
-	char *need_mb;
-};
+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 char __thread need_mb;
-static int num_readers, alloc_readers;
+static LIST_HEAD(registry);
 
-void internal_urcu_lock(void)
+static void internal_urcu_lock(void)
 {
 	int ret;
 
@@ -89,17 +78,12 @@ void internal_urcu_lock(void)
 			perror("Error in pthread mutex lock");
 			exit(-1);
 		}
-		if (need_mb) {
-			smp_mb();
-			need_mb = 0;
-			smp_mb();
-		}
 		poll(NULL,0,10);
 	}
 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 }
 
-void internal_urcu_unlock(void)
+static void internal_urcu_unlock(void)
 {
 	int ret;
 
@@ -110,127 +94,175 @@ void internal_urcu_unlock(void)
 	}
 }
 
-#ifdef DEBUG_FULL_MB
-#ifdef HAS_INCOHERENT_CACHES
-static void force_mb_single_thread(struct reader_registry *index)
+/*
+ * 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);
 }
-#endif /* #ifdef HAS_INCOHERENT_CACHES */
 
-static void force_mb_all_threads(void)
+static void wait_for_quiescent_state(void)
 {
-	smp_mb();
+	LIST_HEAD(qsreaders);
+	int wait_loops = 0;
+	struct urcu_reader *index, *tmp;
+
+	if (list_empty(&registry))
+		return;
+	/*
+	 * Wait for each thread rcu_reader_qs_gp count to become 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 */
+			}
+		}
+	}
+	/* put back the reader list in the registry */
+	list_splice(&qsreaders, &registry);
 }
-#else /* #ifdef DEBUG_FULL_MB */
-#ifdef HAS_INCOHERENT_CACHES
-static void force_mb_single_thread(struct reader_registry *index)
+
+/*
+ * 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)
 {
-	assert(registry);
-	/*
-	 * 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.
+	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.
 	 */
-	*index->need_mb = 1;
-	smp_mc();	/* write ->need_mb before sending the signals */
-	pthread_kill(index->tid, SIGURCU);
+	/* Write new ptr before changing the qparity */
 	smp_mb();
+
 	/*
-	 * Wait for sighandler (and thus mb()) to execute on every thread.
-	 * BUSY-LOOP.
+	 * 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.
 	 */
-	while (*index->need_mb) {
-		poll(NULL, 0, 1);
-	}
-	smp_mb();	/* read ->need_mb before ending the barrier */
-}
-#endif /* #ifdef HAS_INCOHERENT_CACHES */
+	if (was_online)
+		STORE_SHARED(urcu_reader.ctr, 0);
+
+	internal_urcu_lock();
+
+	switch_next_urcu_qparity();	/* 0 -> 1 */
 
-static void force_mb_all_threads(void)
-{
-	struct reader_registry *index;
 	/*
-	 * Ask for each threads to execute a smp_mb() so we can consider the
-	 * compiler barriers around rcu read lock as real memory barriers.
+	 * 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.
 	 */
-	if (!registry)
-		return;
+
 	/*
-	 * 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.
+	 * Wait for previous parity to be empty of readers.
 	 */
-	for (index = registry; index < registry + num_readers; index++) {
-		*index->need_mb = 1;
-		smp_mc();	/* write need_mb before sending the signal */
-		pthread_kill(index->tid, SIGURCU);
-	}
+	wait_for_quiescent_state();	/* Wait readers in parity 0 */
+
 	/*
-	 * Wait for sighandler (and thus mb()) to execute on every thread.
-	 *
-	 * Note that the pthread_kill() will never be executed on systems
-	 * that correctly deliver signals in a timely manner.  However, it
-	 * is not uncommon for kernels to have bugs that can result in
-	 * lost or unduly delayed signals.
-	 *
-	 * If you are seeing the below pthread_kill() executing much at
-	 * all, we suggest testing the underlying kernel and filing the
-	 * relevant bug report.  For Linux kernels, we recommend getting
-	 * the Linux Test Project (LTP).
+	 * 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.
 	 */
-	for (index = registry; index < registry + num_readers; index++) {
-		while (*index->need_mb) {
-			pthread_kill(index->tid, SIGURCU);
-			poll(NULL, 0, 1);
-		}
-	}
-	smp_mb();	/* read ->need_mb before ending the barrier */
-}
-#endif /* #else #ifdef DEBUG_FULL_MB */
 
-void wait_for_quiescent_state(void)
-{
-	struct reader_registry *index;
+	switch_next_urcu_qparity();	/* 1 -> 0 */
 
-	if (!registry)
-		return;
 	/*
-	 * Wait for each thread urcu_active_readers count to become 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.
 	 */
-	for (index = registry; index < registry + num_readers; index++) {
-#ifndef HAS_INCOHERENT_CACHES
-		while (rcu_old_gp_ongoing(index->urcu_active_readers))
-			cpu_relax();
-#else /* #ifndef HAS_INCOHERENT_CACHES */
-		int wait_loops = 0;
-		/*
-		 * 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)) {
-			if (wait_loops++ == KICK_READER_LOOPS) {
-				force_mb_single_thread(index);
-				wait_loops = 0;
-			} else {
-				cpu_relax();
-			}
-		}
-#endif /* #else #ifndef HAS_INCOHERENT_CACHES */
-	}
-}
 
+	/*
+	 * 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)
 {
+	unsigned long was_online;
+
+	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)
+		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)
+		_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.
@@ -254,13 +286,19 @@ void *rcu_dereference(void *p)
 void *rcu_assign_pointer_sym(void **p, void *v)
 {
 	wmb();
-	return STORE_SHARED(p, v);
+	return STORE_SHARED(*p, v);
+}
+
+void *rcu_cmpxchg_pointer_sym(void **p, void *old, void *_new)
+{
+	wmb();
+	return uatomic_cmpxchg(p, old, _new);
 }
 
 void *rcu_xchg_pointer_sym(void **p, void *v)
 {
 	wmb();
-	return xchg(p, v);
+	return uatomic_xchg(p, v);
 }
 
 void *rcu_publish_content_sym(void **p, void *v)
@@ -272,119 +310,45 @@ void *rcu_publish_content_sym(void **p, void *v)
 	return oldptr;
 }
 
-static void rcu_add_reader(pthread_t id)
+void rcu_quiescent_state(void)
 {
-	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].urcu_active_readers = &urcu_active_readers;
-	registry[num_readers].need_mb = &need_mb;
-	num_readers++;
+	_rcu_quiescent_state();
 }
 
-/*
- * Never shrink (implementation limitation).
- * This is O(nb threads). Eventually use a hash table.
- */
-static void rcu_remove_reader(pthread_t id)
+void rcu_thread_offline(void)
 {
-	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].urcu_active_readers = NULL;
-			num_readers--;
-			return;
-		}
-	}
-	/* Hrm not found, forgot to register ? */
-	assert(0);
+	_rcu_thread_offline();
 }
 
-void rcu_register_thread(void)
+void rcu_thread_online(void)
 {
-	internal_urcu_lock();
-	urcu_init();	/* In case gcc does not support constructor attribute */
-	rcu_add_reader(pthread_self());
-	internal_urcu_unlock();
+	_rcu_thread_online();
 }
 
-void rcu_unregister_thread(void)
+void rcu_register_thread(void)
 {
+	urcu_reader.tid = pthread_self();
+	assert(urcu_reader.ctr == 0);
+
 	internal_urcu_lock();
-	rcu_remove_reader(pthread_self());
+	list_add(&urcu_reader.head, &registry);
 	internal_urcu_unlock();
+	_rcu_thread_online();
 }
 
-#ifndef DEBUG_FULL_MB
-static void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
+void rcu_unregister_thread(void)
 {
 	/*
-	 * 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.
+	 * We have to make the thread offline otherwise we end up dealocking
+	 * with a waiting writer.
 	 */
-	smp_mb();
-	need_mb = 0;
-	smp_mb();
-}
-
-/*
- * urcu_init constructor. Called when the library is linked, but also when
- * reader threads are calling rcu_register_thread().
- * Should only be called by a single thread at a given time. This is ensured by
- * holing the internal_urcu_lock() from rcu_register_thread() or by running at
- * library load time, which should not be executed by multiple threads nor
- * concurrently with rcu_register_thread() anyway.
- */
-void urcu_init(void)
-{
-	struct sigaction act;
-	int ret;
-
-	if (init_done)
-		return;
-	init_done = 1;
-
-	act.sa_sigaction = sigurcu_handler;
-	ret = sigaction(SIGURCU, &act, NULL);
-	if (ret) {
-		perror("Error in sigaction");
-		exit(-1);
-	}
+	_rcu_thread_offline();
+	internal_urcu_lock();
+	list_del(&urcu_reader.head);
+	internal_urcu_unlock();
 }
 
-void urcu_exit(void)
+void rcu_exit(void)
 {
-	struct sigaction act;
-	int ret;
-
-	ret = sigaction(SIGURCU, NULL, &act);
-	if (ret) {
-		perror("Error in sigaction");
-		exit(-1);
-	}
-	assert(act.sa_sigaction == sigurcu_handler);
-	free(registry);
+	assert(list_empty(&registry));
 }
-#endif /* #ifndef DEBUG_FULL_MB */