Re: [Qemu-devel] [PATCH 4/9] rcu: introduce RCU-enabled QLIST

[Paolo Bonzini <pbonzini@redhat.com>]

On 04/02/2015 04:42, Fam Zheng wrote:
> On Tue, 02/03 13:52, Paolo Bonzini wrote:
>> From: Mike Day <ncmike@ncultra.org>
>>
>> Add RCU-enabled variants on the existing bsd DQ facility. Each
>> operation has the same interface as the existing (non-RCU)
>> version. Also, each operation is implemented as macro.
>>
>> Using the RCU-enabled QLIST, existing QLIST users will be able to
>> convert to RCU without using a different list interface.
>>
>> Signed-off-by: Mike Day <ncmike@ncultra.org>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>  hw/9pfs/virtio-9p-synth.c |   2 +-
>>  include/qemu/queue.h      |  11 --
>>  include/qemu/rcu_queue.h  | 134 ++++++++++++++++++++
>>  tests/Makefile            |   5 +-
>>  tests/test-rcu-list.c     | 306 ++++++++++++++++++++++++++++++++++++++++++++++
>>  5 files changed, 445 insertions(+), 13 deletions(-)
>>  create mode 100644 include/qemu/rcu_queue.h
>>  create mode 100644 tests/test-rcu-list.c
>>
>> diff --git a/hw/9pfs/virtio-9p-synth.c b/hw/9pfs/virtio-9p-synth.c
>> index e75aa87..a0ab9a8 100644
>> --- a/hw/9pfs/virtio-9p-synth.c
>> +++ b/hw/9pfs/virtio-9p-synth.c
>> @@ -18,7 +18,7 @@
>>  #include "fsdev/qemu-fsdev.h"
>>  #include "virtio-9p-synth.h"
>>  #include "qemu/rcu.h"
>> -
>> +#include "qemu/rcu_queue.h"
>>  #include <sys/stat.h>
>>  
>>  /* Root node for synth file system */
>> diff --git a/include/qemu/queue.h b/include/qemu/queue.h
>> index c602797..8094150 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..3aca7a5
>> --- /dev/null
>> +++ b/include/qemu/rcu_queue.h
>> @@ -0,0 +1,134 @@
>> +#ifndef QEMU_RCU_QUEUE_H
>> +#define QEMU_RCU_QUEUE_H
>> +
>> +/*
>> + * rcu_queue.h
>> + *
>> + * RCU-friendly versions of the queue.h primitives.
>> + *
>> + * 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/queue.h"
>> +#include "qemu/atomic.h"
>> +
>> +#ifdef __cplusplus
>> +extern "C" {
>> +#endif
>> +
>> +
>> +/*
>> + * List access methods.
>> + */
>> +#define QLIST_EMPTY_RCU(head) (atomic_rcu_read(&(head)->lh_first) == NULL)
>> +#define QLIST_FIRST_RCU(head) (atomic_rcu_read(&(head)->lh_first))
>> +#define QLIST_NEXT_RCU(elm, field) (atomic_rcu_read(&(elm)->field.le_next))
>> +
>> +/*
>> + * 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;                       \
>> +    }                                               \
>> +    *(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) = atomic_rcu_read(&(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) = atomic_rcu_read(&(var)->field.le_next), 1);  \
>> +           (var) = (next_var))
>> +
>> +#ifdef __cplusplus
>> +}
>> +#endif
>> +#endif /* QEMU_RCU_QUEUE.H */
>> diff --git a/tests/Makefile b/tests/Makefile
>> index db5b3c3..ef8d589 100644
>> --- a/tests/Makefile
>> +++ b/tests/Makefile
>> @@ -62,6 +62,8 @@ check-unit-y += tests/test-int128$(EXESUF)
>>  gcov-files-test-int128-y =
>>  check-unit-y += tests/rcutorture$(EXESUF)
>>  gcov-files-rcutorture-y = util/rcu.c
>> +check-unit-y += tests/test-rcu-list$(EXESUF)
>> +gcov-files-test-rcu-list-y = util/rcu.c
>>  check-unit-y += tests/test-bitops$(EXESUF)
>>  check-unit-$(CONFIG_HAS_GLIB_SUBPROCESS_TESTS) += tests/test-qdev-global-props$(EXESUF)
>>  check-unit-y += tests/check-qom-interface$(EXESUF)
>> @@ -226,7 +228,7 @@ test-obj-y = tests/check-qint.o tests/check-qstring.o tests/check-qdict.o \
>>  	tests/test-qmp-commands.o tests/test-visitor-serialization.o \
>>  	tests/test-x86-cpuid.o tests/test-mul64.o tests/test-int128.o \
>>  	tests/test-opts-visitor.o tests/test-qmp-event.o \
>> -	tests/rcutorture.o
>> +	tests/rcutorture.o tests/test-rcu-list.o
>>  
>>  test-qapi-obj-y = tests/test-qapi-visit.o tests/test-qapi-types.o \
>>  		  tests/test-qapi-event.o
>> @@ -256,6 +258,7 @@ tests/test-xbzrle$(EXESUF): tests/test-xbzrle.o migration/xbzrle.o page_cache.o
>>  tests/test-cutils$(EXESUF): tests/test-cutils.o util/cutils.o
>>  tests/test-int128$(EXESUF): tests/test-int128.o
>>  tests/rcutorture$(EXESUF): tests/rcutorture.o libqemuutil.a
>> +tests/test-rcu-list$(EXESUF): tests/test-rcu-list.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/hotplug.o\
>> diff --git a/tests/test-rcu-list.c b/tests/test-rcu-list.c
>> new file mode 100644
>> index 0000000..5ce55e9
>> --- /dev/null
>> +++ b/tests/test-rcu-list.c
>> @@ -0,0 +1,306 @@
>> +/*
>> + * 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.
>> + */
> 
> Move below the includes?
> 
>> +
>> +#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/osdep.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 g_test_in_charge = 0;
>> +
>> +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], "test", 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 {
>> +    QLIST_ENTRY(list_element) entry;
>> +    struct rcu_head rcu;
>> +    long long val;
>> +};
>> +
>> +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)
>> +{
>> +    long long j, n_reads_local = 0;
>> +    struct list_element *el;
>> +
>> +    *(struct rcu_reader_data **)arg = &rcu_reader;
>> +    atomic_inc(&nthreadsrunning);
>> +    while (goflag == GOFLAG_INIT) {
>> +        g_usleep(1000);
>> +    }
>> +
>> +    while (goflag == GOFLAG_RUN) {
>> +        rcu_read_lock();
>> +        QLIST_FOREACH_RCU(el, &Q_list_head, entry) {
>> +            j = atomic_read(&el->val);
>> +            (void)j;
>> +            n_reads_local++;
>> +            if (goflag == GOFLAG_STOP) {
>> +                break;
>> +            }
>> +        }
>> +        rcu_read_unlock();
>> +
>> +        g_usleep(100);
>> +    }
>> +    atomic_add(&n_reads, n_reads_local);
>> +    return NULL;
>> +}
>> +
>> +
>> +static void *rcu_q_updater(void *arg)
>> +{
>> +    int 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 = &rcu_reader;
>> +    atomic_inc(&nthreadsrunning);
>> +    while (goflag == GOFLAG_INIT) {
>> +        g_usleep(1000);
>> +    }
>> +
>> +    while (goflag == GOFLAG_RUN) {
>> +        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();
>> +    }
>> +    synchronize_rcu();
> 
> Is one of two synchronize_rcu's superfluous? Otherwise looks good:

There is a "break"s in the loop, in that case the second synchronize_rcu
is necessary.

Paolo

> 
> Reviewed-by: Fam Zheng <famz@redhat.com>
> 
>> +    atomic_add(&n_updates, n_updates_local);
>> +    atomic_add(&n_nodes_removed, n_removed_local);
>> +    return NULL;
>> +}
> 
> <...>
> 
>