[Qemu-devel] [RFC PATCH] Introduce RCU-enabled DQs.

[Mike Day]

Add RCU-enabled variants on the existing bsd DQ facility. Each Q
operation has the same interface as the existing (non-RCU)
version. Also, each operation is implemented as macro for now.

Using the RCU-enabled DQ, existing DQ users will be able to convert to
RCU without using a different list interface.

To accompany the RCU-enabled DQ, there is also a test file that uses
concurrent readers to contend with a single updater.

This patchset builds on top of Paolo Bonzini's rcu tree:
https://github.com/bonzini/qemu/tree/rcu

Signed-off-by: Mike Day <address@hidden>
---
 docs/rcu.txt             |   2 +-
 include/qemu/queue.h     |  11 --
 include/qemu/rcu_queue.h | 137 +++++++++++++++++++++++
 tests/Makefile           |  14 ++-
 tests/rcuq_test.c        | 279 +++++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 430 insertions(+), 13 deletions(-)

diff --git a/docs/rcu.txt b/docs/rcu.txt
index b3c593c..de59896 100644
--- a/docs/rcu.txt
+++ b/docs/rcu.txt
@@ -106,7 +106,7 @@ The core RCU API is small:
         so that the reclaimer function can fetch the struct foo address
         and free it:
 
-            call_rcu1(foo_reclaim, &foo.rcu);
+            call_rcu1(&foo.rcu, foo_reclaim);
 
             void foo_reclaim(struct rcu_head *rp)
             {
diff --git a/include/qemu/queue.h b/include/qemu/queue.h
index 847ddd1..f6f0636 100644
--- a/include/qemu/queue.h
+++ b/include/qemu/queue.h
@@ -139,17 +139,6 @@ struct {                                                   
             \
         (elm)->field.le_prev = &(head)->lh_first;                       \
 } while (/*CONSTCOND*/0)
 
-#define QLIST_INSERT_HEAD_RCU(head, elm, field) do {                    \
-        (elm)->field.le_prev = &(head)->lh_first;                       \
-        (elm)->field.le_next = (head)->lh_first;                        \
-        smp_wmb(); /* fill elm before linking it */                     \
-        if ((head)->lh_first != NULL)  {                                \
-            (head)->lh_first->field.le_prev = &(elm)->field.le_next;    \
-        }                                                               \
-        (head)->lh_first = (elm);                                       \
-        smp_wmb();                                                      \
-} while (/* CONSTCOND*/0)
-
 #define QLIST_REMOVE(elm, field) do {                                   \
         if ((elm)->field.le_next != NULL)                               \
                 (elm)->field.le_next->field.le_prev =                   \
diff --git a/include/qemu/rcu_queue.h b/include/qemu/rcu_queue.h
new file mode 100644
index 0000000..e82196d
--- /dev/null
+++ b/include/qemu/rcu_queue.h
@@ -0,0 +1,137 @@
+#ifndef QEMU_RCU_SYS_QUEUE_H
+#define QEMU_RCU_SYS_QUEUE_H
+
+/*
+ * rc_queue.h
+ *
+ * Userspace RCU QSBR header.
+ *
+ * LGPL-compatible code should include this header with :
+ *
+ * #define _LGPL_SOURCE
+ * #include <urcu.h>
+ *
+ * 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
+ *
+ * Copyright (c) 2013 Mike D. Day, IBM Corporation.
+ *
+ * IBM's contributions to this file may be relicensed under LGPLv2 or later.
+ */
+
+#include "qemu/rcu.h"  /* rcu.h includes qemu/queue.h and qemu/atomic.h */
+
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*
+ * List functions.
+ */
+
+
+/* 
+ *  The difference between atomic_read/set and atomic_rcu_read/set
+ *  is in the including of a read/write memory barrier to the volatile
+ *  access. atomic_rcu_* macros include the memory barrier, the 
+ *  plain atomic macros do not. Therefore, it should be correct to 
+ *  issue a series of reads or writes to the same element using only 
+ *  the atomic_* macro, until the last read or write, which should be 
+ *  atomic_rcu_* to introduce a read or write memory barrier as 
+ *  appropriate.  
+ */
+
+/* Upon publication of the listelm->next value, list readers
+ * will see the new node when following next pointers from 
+ * antecedent nodes, but may not see the new node when following 
+ * prev pointers from subsequent nodes until after the rcu grace 
+ * period expires.   
+ * see linux/include/rculist.h __list_add_rcu(new, prev, next)
+ */
+#define QLIST_INSERT_AFTER_RCU(listelm, elm, field) do {    \
+    (elm)->field.le_next = (listelm)->field.le_next;        \
+    (elm)->field.le_prev = &(listelm)->field.le_next;       \
+    atomic_rcu_set(&(listelm)->field.le_next, (elm));       \
+    if ((elm)->field.le_next != NULL) {                     \
+       (elm)->field.le_next->field.le_prev =                \
+        &(elm)->field.le_next;                              \
+        }                                                   \
+} while (/*CONSTCOND*/0)
+    
+/* Upon publication of the listelm->prev->next value, list 
+ * readers will see the new element when following prev pointers 
+ * from subsequent elements, but may not see the new element 
+ * when following next pointers from antecedent elements 
+ * until after the rcu grace period expires. 
+ */
+
+#define QLIST_INSERT_BEFORE_RCU(listelm, elm, field) do {   \
+        (elm)->field.le_prev = (listelm)->field.le_prev;    \
+    (elm)->field.le_next = (listelm);                       \
+    atomic_rcu_set((listelm)->field.le_prev, (elm));        \
+    (listelm)->field.le_prev = &(elm)->field.le_next;       \
+} while (/*CONSTCOND*/0)
+
+
+/* Upon publication of the head->first value, list readers
+ * will see the new element when following the head, but may
+ * not see the new element when following prev pointers from
+ * subsequent elements until after the rcu grace period has 
+ * expired. 
+ */
+
+#define QLIST_INSERT_HEAD_RCU(head, elm, field) do {    \
+    (elm)->field.le_prev = &(head)->lh_first;           \
+    (elm)->field.le_next = (head)->lh_first;            \
+    atomic_rcu_set((&(head)->lh_first), (elm));         \
+    if ((elm)->field.le_next != NULL) {                 \
+       (elm)->field.le_next->field.le_prev =            \
+        &(elm)->field.le_next;                          \
+    }                                                   \
+} while (/*CONSTCOND*/0)
+    
+
+/* prior to publication of the elm->prev->next value, some list 
+ * readers  may still see the removed element when following
+ * the antecedent's next pointer. 
+ */
+
+#define QLIST_REMOVE_RCU(elm, field) do {                       \
+    if ((elm)->field.le_next != NULL) {                         \
+       (elm)->field.le_next->field.le_prev =                    \
+        (elm)->field.le_prev;                                   \
+    }                                                           \
+    atomic_rcu_set((elm)->field.le_prev, (elm)->field.le_next); \
+} while (/*CONSTCOND*/0)
+
+/* list traversal must occur within an rcu critical section. 
+ */
+
+#define QLIST_FOREACH_RCU(var, head, field)                 \
+        for ((var) = atomic_rcu_read(&(head)->lh_first);    \
+                (var);                                      \
+                (var) = ((var)->field.le_next))
+
+/* list traversal must occur within an rcu critical section. 
+ */
+#define QLIST_FOREACH_SAFE_RCU(var, head, field, next_var)  \
+    for ((var) = (atomic_rcu_read(&(head)->lh_first));      \
+      (var) && ((next_var) = (var)->field.le_next, 1);      \
+           (var) = (next_var))
+
+#ifdef __cplusplus
+}   
+#endif
+#endif /* QEMU_RCU_SYS_QUEUE.H */
diff --git a/tests/Makefile b/tests/Makefile
index 136b7a4..49149ef 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -43,6 +43,8 @@ gcov-files-test-xbzrle-y = xbzrle.c
 check-unit-y += tests/test-cutils$(EXESUF)
 gcov-files-test-cutils-y += util/cutils.c
 check-unit-y += tests/test-mul64$(EXESUF)
+check-unit-y += tests/test-tls$(EXESUF)
+# all code tested by test-tls is inside tls.h
 gcov-files-test-mul64-y = util/host-utils.c
 check-unit-y += tests/test-tls$(EXESUF)
 # all code tested by test-tls is inside tls.h
@@ -53,6 +55,9 @@ gcov-files-test-int128-y =
 check-unit-y += tests/test-bitops$(EXESUF)
 check-unit-y += tests/rcutorture$(EXESUF)
 gcov-files-rcutorture-y = util/rcu.c
+check-unit-y += tests/rcuq_test$(EXESUF)
+gcov-files-rcuq_test-y = util/rcu.c
+check-unit-y += tests/test-qdev-global-props$(EXESUF)
 
 check-block-$(CONFIG_POSIX) += tests/qemu-iotests-quick.sh
 
@@ -105,7 +110,7 @@ test-obj-y = tests/check-qint.o tests/check-qstring.o 
tests/check-qdict.o \
        tests/test-qmp-input-visitor.o tests/test-qmp-input-strict.o \
        tests/test-qmp-commands.o tests/test-visitor-serialization.o \
        tests/test-x86-cpuid.o tests/test-mul64.o tests/test-int128.o \
-       tests/test-tls.o tests/rcutorture.o
+       tests/test-tls.o tests/rcutorture.o tests/rcuq_test.o
 
 test-qapi-obj-y = tests/test-qapi-visit.o tests/test-qapi-types.o
 
@@ -131,6 +136,13 @@ tests/test-cutils$(EXESUF): tests/test-cutils.o 
util/cutils.o
 tests/test-int128$(EXESUF): tests/test-int128.o
 tests/test-tls$(EXESUF): tests/test-tls.o libqemuutil.a
 tests/rcutorture$(EXESUF): tests/rcutorture.o libqemuutil.a
+tests/rcuq_test$(EXESUF): tests/rcuq_test.o libqemuutil.a
+tests/test-qdev-global-props$(EXESUF): tests/test-qdev-global-props.o \
+       hw/core/qdev.o hw/core/qdev-properties.o \
+       hw/core/irq.o \
+       qom/object.o qom/container.o qom/qom-qobject.o \
+       $(test-qapi-obj-y) \
+       libqemuutil.a libqemustub.a
 
 tests/test-qapi-types.c tests/test-qapi-types.h :\
 $(SRC_PATH)/tests/qapi-schema/qapi-schema-test.json 
$(SRC_PATH)/scripts/qapi-types.py
diff --git a/tests/rcuq_test.c b/tests/rcuq_test.c
new file mode 100644
index 0000000..b5ce36d
--- /dev/null
+++ b/tests/rcuq_test.c
@@ -0,0 +1,279 @@
+/*
+ * rcuq_test.c 
+ * 
+ * usage: rcuq_test <readers> <duration>
+ * 
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * Copyright (c) 2013 Mike D. Day, IBM Corporation.
+ */
+
+/*
+ * Test variables.
+ */
+
+#include <glib.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include "qemu/atomic.h"
+#include "qemu/rcu.h"
+#include "qemu/compiler.h"
+#include "qemu/thread.h"
+#include "qemu/rcu_queue.h"
+
+long long n_reads = 0LL;
+long long n_updates = 0LL;
+long long n_reclaims = 0LL;
+long long n_nodes_removed = 0LL;
+long long n_nodes = 0LL;
+
+
+int nthreadsrunning;
+
+char argsbuf[64];
+
+#define GOFLAG_INIT 0
+#define GOFLAG_RUN  1
+#define GOFLAG_STOP 2
+
+static volatile int goflag = GOFLAG_INIT;
+
+#define RCU_READ_RUN 1000
+#define RCU_UPDATE_RUN 10
+#define NR_THREADS 100
+#define RCU_Q_LEN 100
+
+static QemuThread threads[NR_THREADS];
+static struct rcu_reader_data *data[NR_THREADS];
+static int n_threads;
+
+static int select_random_el(int max)
+{
+    return (rand() % max);
+}
+
+
+static void create_thread(void *(*func)(void *))
+{
+    if (n_threads >= NR_THREADS) {
+        fprintf(stderr, "Thread limit of %d exceeded!\n", NR_THREADS);
+        exit(-1);
+    }
+    qemu_thread_create(&threads[n_threads], func, &data[n_threads],
+                       QEMU_THREAD_JOINABLE);
+    n_threads++;
+}
+
+static void wait_all_threads(void)
+{
+    int i;
+
+    for (i = 0; i < n_threads; i++) {
+        qemu_thread_join(&threads[i]);
+    }
+    n_threads = 0;
+}
+
+
+struct list_element {
+    struct rcu_head rcu;
+    long long val;
+    QLIST_ENTRY(list_element) entry;
+};
+
+static void reclaim_list_el(struct rcu_head *prcu) 
+{
+    struct list_element *el = container_of(prcu, struct list_element, rcu);
+    g_free(el);
+    atomic_add(&n_reclaims, 1);
+}
+
+static QLIST_HEAD(q_list_head, list_element) Q_list_head;
+
+
+                                             static void *rcu_q_reader(void 
*arg) 
+{
+    int i, j;
+    long long n_reads_local = 0;
+    struct list_element *el;
+
+    *(struct rcu_reader_data **)arg = tls_get_rcu_reader();
+    atomic_inc(&nthreadsrunning);
+    rcu_thread_offline();
+    while (goflag == GOFLAG_INIT) {
+        g_usleep(1000);
+    }
+
+    rcu_thread_online();    
+    while (goflag == GOFLAG_RUN) {
+        for (i = 0; i < RCU_READ_RUN && goflag == GOFLAG_RUN; i++) {
+            rcu_read_lock();
+            QLIST_FOREACH_RCU(el, &Q_list_head, entry) {
+                j = el->val;
+                if (j){};
+            }
+            rcu_read_unlock();
+        }
+        n_reads_local += RCU_READ_RUN;
+        rcu_quiescent_state();
+        if (goflag == GOFLAG_STOP)
+            break;
+        g_usleep(10);
+    }
+    rcu_thread_offline();
+    atomic_add(&n_reads, n_reads_local);
+    return NULL;
+}
+
+
+static void *rcu_q_updater(void *arg)
+{
+    int i, j, target_el;
+    long long n_updates_local = 0;
+    long long n_removed_local = 0;
+    struct list_element *el, *prev_el;
+
+    *(struct rcu_reader_data **)arg = tls_get_rcu_reader();
+    atomic_inc(&nthreadsrunning);
+    while (goflag == GOFLAG_INIT) {
+        g_usleep(1000);
+    }
+
+    while (goflag == GOFLAG_RUN) {
+        for (i = 0; i < RCU_UPDATE_RUN && goflag == GOFLAG_RUN; i++) {
+            target_el = select_random_el(RCU_Q_LEN);
+            j = 0;
+            /* FOREACH_RCU could work here but let's use both macros */
+            QLIST_FOREACH_SAFE_RCU(prev_el, &Q_list_head, entry, el) {
+                j++;
+                if (target_el <= j) {
+                    QLIST_REMOVE_RCU(prev_el, entry);
+                    /* may be more than one updater in the future */
+                    call_rcu1(&prev_el->rcu, reclaim_list_el);
+                    n_removed_local++;
+                    break;
+                }
+            }
+            if (goflag == GOFLAG_STOP)
+                break;
+            target_el = select_random_el(RCU_Q_LEN);
+            j = 0;
+            QLIST_FOREACH_RCU(el, &Q_list_head, entry) {
+                j++;
+                if (target_el <= j) {
+                    prev_el = g_new(struct list_element, 1);
+                    atomic_add(&n_nodes, 1);
+                    prev_el->val = atomic_read(&n_nodes);
+                    QLIST_INSERT_BEFORE_RCU(el, prev_el, entry);
+                    break;
+                }
+            }
+        }
+        n_updates_local += 2;
+        synchronize_rcu();
+    }
+    rcu_quiescent_state();
+    atomic_add(&n_updates, n_updates_local);
+    atomic_add(&n_nodes_removed, n_removed_local);
+    return NULL;
+}
+
+static void rcu_qtest_init(void)
+{
+    struct list_element *new_el;
+    int i;
+    nthreadsrunning = 0;
+    srand(time(0)); 
+    for (i = 0; i < RCU_Q_LEN; i++) {   
+        new_el = g_new(struct list_element, 1);
+        new_el->val = i;
+        QLIST_INSERT_HEAD_RCU(&Q_list_head, new_el, entry);
+    }
+    atomic_add(&n_nodes, RCU_Q_LEN);
+}
+
+static void rcu_qtest_run(int duration, int nreaders)
+{
+    int nthreads = nreaders + 1;
+    while (atomic_read(&nthreadsrunning) < nthreads) {
+        g_usleep(1000);
+    }
+
+    goflag = GOFLAG_RUN;
+    sleep(duration);
+    goflag = GOFLAG_STOP;
+    wait_all_threads();
+}
+
+
+static void rcu_qtest(const char *test, int duration, int nreaders) 
+{
+    int i;
+    long long n_removed_local=0;
+    
+    struct list_element *el, *prev_el;
+
+    rcu_qtest_init();
+    for (i = 0; i < nreaders; i++) {
+        create_thread(rcu_q_reader);
+    }
+    create_thread(rcu_q_updater);
+    rcu_qtest_run(duration, nreaders);
+    rcu_thread_offline();
+    
+    QLIST_FOREACH_SAFE_RCU(prev_el, &Q_list_head, entry, el) {
+        QLIST_REMOVE_RCU(prev_el, entry);
+        call_rcu1(&prev_el->rcu, reclaim_list_el);
+        n_removed_local++;
+    }
+    atomic_add(&n_nodes_removed, n_removed_local);
+    rcu_quiescent_state();
+    synchronize_rcu();
+    while (n_nodes_removed > n_reclaims){
+        rcu_quiescent_state();
+        g_usleep(100);
+        synchronize_rcu();
+    }
+    printf("%s: %d readers; 1 updater; nodes read: %lld, nodes removed: %lld; 
nodes reclaimed: %lld\n", 
+           test, nthreadsrunning -1, n_reads, n_nodes_removed, n_reclaims);
+    exit(0);
+}
+
+static void usage(int argc, char *argv[])
+{
+    fprintf(stderr, "Usage: %s duration nreaders\n", argv[0]);
+    exit(-1);
+}
+
+
+int main(int argc, char *argv[])
+{
+    int duration = 0, readers = 0;
+    
+    if (argc >= 2) {
+        duration = strtoul(argv[1], NULL, 0);
+    }
+    if (argc >= 3) {
+        readers = strtoul(argv[2], NULL, 0);    
+    }
+
+    /* This thread is not part of the test.  */
+    rcu_thread_offline();
+    if (duration && readers)
+        rcu_qtest(argv[0], duration, readers);
+    usage(argc, argv);
+    return 0;
+}
-- 
1.8.3.1