X-Git-Url: https://git.lttng.org/?p=urcu.git;a=blobdiff_plain;f=urcu.c;h=c13ad634e7cb7b98f24d8c9a5a756eb26487bf7b;hp=9fde6238f695b06db846866a704f9ed595bd6d0c;hb=efafc824ca148c6b2ca7f95dc86d2aaa38d4a923;hpb=f4a486ac095bd844d0c18995737d589b8f085b69

diff --git a/urcu.c b/urcu.c
index 9fde623..c13ad63 100644
--- a/urcu.c
+++ b/urcu.c
@@ -3,9 +3,24 @@
  *
  * Userspace RCU library
  *
- * Copyright February 2009 - Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
+ * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
+ * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
  *
- * Distributed under GPLv2
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ * IBM's contributions to this file may be relicensed under LGPLv2 or later.
  */
 
 #include <stdio.h>
@@ -14,44 +29,77 @@
 #include <assert.h>
 #include <stdlib.h>
 #include <string.h>
+#include <errno.h>
+#include <poll.h>
 
+#include "urcu-static.h"
+/* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
 #include "urcu.h"
 
-pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
+#ifndef URCU_MB
+static int init_done;
 
-/* Global grace period counter */
-int urcu_gp_ctr;
+void __attribute__((constructor)) urcu_init(void);
+void __attribute__((destructor)) urcu_exit(void);
+#else
+void urcu_init(void)
+{
+}
+#endif
 
-int __thread urcu_active_readers;
+static pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
 
-/* Thread IDs of registered readers */
-#define INIT_NUM_THREADS 4
+int gp_futex;
 
-struct reader_data {
-	pthread_t tid;
-	int *urcu_active_readers;
-};
+/*
+ * Global grace period counter.
+ * Contains the current RCU_GP_CTR_BIT.
+ * Also has a RCU_GP_COUNT 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;
+
+/*
+ * Written to only by each individual reader. Read by both the reader and the
+ * writers.
+ */
+struct urcu_reader __thread urcu_reader;
 
 #ifdef DEBUG_YIELD
 unsigned int yield_active;
 unsigned int __thread rand_yield;
 #endif
 
-static struct reader_data *reader_data;
-static int num_readers, alloc_readers;
-static int sig_done;
+static LIST_HEAD(registry);
 
-void internal_urcu_lock(void)
+static void internal_urcu_lock(void)
 {
 	int ret;
+
+#ifndef DISTRUST_SIGNALS_EXTREME
 	ret = pthread_mutex_lock(&urcu_mutex);
 	if (ret) {
 		perror("Error in pthread mutex lock");
 		exit(-1);
 	}
+#else /* #ifndef DISTRUST_SIGNALS_EXTREME */
+	while ((ret = pthread_mutex_trylock(&urcu_mutex)) != 0) {
+		if (ret != EBUSY && ret != EINTR) {
+			printf("ret = %d, errno = %d\n", ret, errno);
+			perror("Error in pthread mutex lock");
+			exit(-1);
+		}
+		if (urcu_reader.need_mb) {
+			smp_mb();
+			urcu_reader.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;
 
@@ -67,164 +115,347 @@ 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 URCU_MB
+#if 0 /* unused */
+static void force_mb_single_thread(struct urcu_reader *index)
+{
+	smp_mb();
+}
+#endif //0
+
+static void force_mb_all_threads(void)
+{
+	smp_mb();
+}
+#else /* #ifdef URCU_MB */
+#if 0 /* unused */
+static void force_mb_single_thread(struct urcu_reader *index)
+{
+	assert(!list_empty(&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.
+	 */
+	index->need_mb = 1;
+	smp_mc();	/* write ->need_mb before sending the signals */
+	pthread_kill(index->tid, SIGURCU);
+	smp_mb();
+	/*
+	 * Wait for sighandler (and thus mb()) to execute on every thread.
+	 * BUSY-LOOP.
+	 */
+	while (index->need_mb) {
+		poll(NULL, 0, 1);
+	}
+	smp_mb();	/* read ->need_mb before ending the barrier */
 }
+#endif //0
 
 static void force_mb_all_threads(void)
 {
-	struct reader_data *index;
+	struct urcu_reader *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)
+	if (list_empty(&registry))
 		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.
+	 */
+	list_for_each_entry(index, &registry, head) {
+		index->need_mb = 1;
+		smp_mc();	/* write need_mb before sending the signal */
 		pthread_kill(index->tid, SIGURCU);
-		debug_yield_write();
 	}
 	/*
 	 * Wait for sighandler (and thus mb()) to execute on every thread.
-	 * BUSY-LOOP.
+	 *
+	 * 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).
 	 */
-	while (sig_done < num_readers)
-		barrier();
-	debug_yield_write();
-	mb();	/* read sig_done before ending the barrier */
-	debug_yield_write();
+	list_for_each_entry(index, &registry, head) {
+		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 URCU_MB */
+
+/*
+ * synchronize_rcu() waiting. Single thread.
+ */
+static void wait_gp(void)
+{
+	/* Read reader_gp before read futex */
+	force_mb_all_threads();
+	if (uatomic_read(&gp_futex) == -1)
+		futex(&gp_futex, FUTEX_WAIT, -1,
+		      NULL, NULL, 0);
 }
 
 void wait_for_quiescent_state(void)
 {
-	struct reader_data *index;
+	LIST_HEAD(qsreaders);
+	int wait_loops = 0;
+	struct urcu_reader *index, *tmp;
 
-	if (!reader_data)
+	if (list_empty(&registry))
 		return;
-	/* Wait for each thread urcu_active_readers count to become 0.
+	/*
+	 * Wait for each thread urcu_reader.ctr count to become 0.
 	 */
-	for (index = reader_data; index < reader_data + num_readers; index++) {
+	for (;;) {
+		wait_loops++;
+		if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
+			uatomic_dec(&gp_futex);
+			/* Write futex before read reader_gp */
+			force_mb_all_threads();
+		}
+
+		list_for_each_entry_safe(index, tmp, &registry, head) {
+			if (!rcu_old_gp_ongoing(&index->ctr))
+				list_move(&index->head, &qsreaders);
+		}
+
+#ifndef HAS_INCOHERENT_CACHES
+		if (list_empty(&registry)) {
+			if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
+				/* Read reader_gp before write futex */
+				force_mb_all_threads();
+				uatomic_set(&gp_futex, 0);
+			}
+			break;
+		} else {
+			if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
+				wait_gp();
+			else
+				cpu_relax();
+		}
+#else /* #ifndef HAS_INCOHERENT_CACHES */
 		/*
-		 * BUSY-LOOP.
+		 * BUSY-LOOP. Force the reader thread to commit its
+		 * urcu_reader.ctr update to memory if we wait for too long.
 		 */
-		while (rcu_old_gp_ongoing(index->urcu_active_readers))
-			barrier();
+		if (list_empty(&registry)) {
+			if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
+				/* Read reader_gp before write futex */
+				force_mb_all_threads();
+				uatomic_set(&gp_futex, 0);
+			}
+			break;
+		} else {
+			switch (wait_loops) {
+			case RCU_QS_ACTIVE_ATTEMPTS:
+				wait_gp();
+				break; /* only escape switch */
+			case KICK_READER_LOOPS:
+				force_mb_all_threads();
+				wait_loops = 0;
+				break; /* only escape switch */
+			default:
+				cpu_relax();
+			}
+		}
+#endif /* #else #ifndef HAS_INCOHERENT_CACHES */
 	}
-	/*
-	 * 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();
+	/* put back the reader list in the registry */
+	list_splice(&qsreaders, &registry);
 }
 
-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.
+	 */
+
+	/*
+	 * Adding a smp_mb() which is _not_ formally required, but makes the
+	 * model easier to understand. It does not have a big performance impact
+	 * anyway, given this is the write-side.
+	 */
+	smp_mb();
 
 	/*
 	 * Wait for previous parity to be empty of readers.
 	 */
-	wait_for_quiescent_state();
+	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.
+	 */
+
+	/*
+	 * Adding a smp_mb() which is _not_ formally required, but makes the
+	 * model easier to understand. It does not have a big performance impact
+	 * anyway, given this is the write-side.
+	 */
+	smp_mb();
+
+	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.
+	 */
+
+	/*
+	 * Adding a smp_mb() which is _not_ formally required, but makes the
+	 * model easier to understand. It does not have a big performance impact
+	 * anyway, given this is the write-side.
+	 */
+	smp_mb();
+
+	/*
+	 * 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 synchronize_rcu(void)
+/*
+ * library wrappers to be used by non-LGPL compatible source code.
+ */
+
+void rcu_read_lock(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();
+	_rcu_read_lock();
 }
 
-void urcu_add_reader(pthread_t id)
+void rcu_read_unlock(void)
 {
-	struct reader_data *oldarray;
+	_rcu_read_unlock();
+}
 
-	if (!reader_data) {
-		alloc_readers = INIT_NUM_THREADS;
-		num_readers = 0;
-		reader_data =
-			malloc(sizeof(struct reader_data) * alloc_readers);
-	}
-	if (alloc_readers < num_readers + 1) {
-		oldarray = reader_data;
-		reader_data = malloc(sizeof(struct reader_data)
-				* (alloc_readers << 1));
-		memcpy(reader_data, oldarray,
-			sizeof(struct reader_data) * alloc_readers);
-		alloc_readers <<= 1;
-		free(oldarray);
-	}
-	reader_data[num_readers].tid = id;
-	/* reference to the TLS of _this_ reader thread. */
-	reader_data[num_readers].urcu_active_readers = &urcu_active_readers;
-	num_readers++;
+void *rcu_dereference(void *p)
+{
+	return _rcu_dereference(p);
 }
 
-/*
- * Never shrink (implementation limitation).
- * This is O(nb threads). Eventually use a hash table.
- */
-void urcu_remove_reader(pthread_t id)
-{
-	struct reader_data *index;
-
-	assert(reader_data != NULL);
-	for (index = reader_data; index < reader_data + num_readers; index++) {
-		if (pthread_equal(index->tid, id)) {
-			memcpy(index, &reader_data[num_readers - 1],
-				sizeof(struct reader_data));
-			reader_data[num_readers - 1].tid = 0;
-			reader_data[num_readers - 1].urcu_active_readers = NULL;
-			num_readers--;
-			return;
-		}
-	}
-	/* Hrm not found, forgot to register ? */
-	assert(0);
+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 uatomic_xchg(p, v);
 }
 
-void urcu_register_thread(void)
+void *rcu_cmpxchg_pointer_sym(void **p, void *old, void *_new)
 {
+	wmb();
+	return uatomic_cmpxchg(p, old, _new);
+}
+
+void *rcu_publish_content_sym(void **p, void *v)
+{
+	void *oldptr;
+
+	oldptr = _rcu_xchg_pointer(p, v);
+	synchronize_rcu();
+	return oldptr;
+}
+
+void rcu_register_thread(void)
+{
+	urcu_reader.tid = pthread_self();
+	assert(urcu_reader.need_mb == 0);
+	assert(urcu_reader.ctr == 0);
+
 	internal_urcu_lock();
-	urcu_add_reader(pthread_self());
+	urcu_init();	/* In case gcc does not support constructor attribute */
+	list_add(&urcu_reader.head, &registry);
 	internal_urcu_unlock();
 }
 
-void urcu_unregister_thread(void)
+void rcu_unregister_thread(void)
 {
 	internal_urcu_lock();
-	urcu_remove_reader(pthread_self());
+	list_del(&urcu_reader.head);
 	internal_urcu_unlock();
 }
 
-void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
+#ifndef URCU_MB
+static void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
 {
-	mb();
-	atomic_inc(&sig_done);
+	/*
+	 * 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();
+	urcu_reader.need_mb = 0;
+	smp_mb();
 }
 
-void __attribute__((constructor)) urcu_init(void)
+/*
+ * 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;
+	act.sa_flags = SA_SIGINFO | SA_RESTART;
+	sigemptyset(&act.sa_mask);
 	ret = sigaction(SIGURCU, &act, NULL);
 	if (ret) {
 		perror("Error in sigaction");
@@ -232,7 +463,7 @@ void __attribute__((constructor)) urcu_init(void)
 	}
 }
 
-void __attribute__((destructor)) urcu_exit(void)
+void urcu_exit(void)
 {
 	struct sigaction act;
 	int ret;
@@ -243,5 +474,6 @@ void __attribute__((destructor)) urcu_exit(void)
 		exit(-1);
 	}
 	assert(act.sa_sigaction == sigurcu_handler);
-	free(reader_data);
+	assert(list_empty(&registry));
 }
+#endif /* #ifndef URCU_MB */