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membarrier: introduce qemu/sys_membarrier.h 

This new header file provides heavy-weight "global" memory barriers that
enforce memory ordering on each running thread belonging to the current
process.  For now, use a dummy implementation that issues memory barriers
on both sides (matching what QEMU has been doing so far).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2018-02-16 10:04:18 +01:00
parent 77a8b8462b
commit c8d3877e48
3 changed files with 26 additions and 7 deletions
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7
include/qemu/rcu.h
View File

@ -27,6 +27,7 @@
#include "qemu/thread.h"
#include "qemu/queue.h"
#include "qemu/atomic.h"
#include "qemu/sys_membarrier.h"
#ifdef __cplusplus
extern "C" {
@ -82,7 +83,7 @@ static inline void rcu_read_lock(void)
atomic_set(&p_rcu_reader->ctr, ctr);
/* Write p_rcu_reader->ctr before reading RCU-protected pointers. */
smp_mb();
smp_mb_placeholder();
}
static inline void rcu_read_unlock(void)
@ -96,13 +97,13 @@ static inline void rcu_read_unlock(void)
/* Ensure that the critical section is seen to precede the
* store to p_rcu_reader->ctr. Together with the following
* smp_mb(), this ensures writes to p_rcu_reader->ctr
* smp_mb_placeholder(), this ensures writes to p_rcu_reader->ctr
* are sequentially consistent.
*/
atomic_store_release(&p_rcu_reader->ctr, 0);
/* Write p_rcu_reader->ctr before reading p_rcu_reader->waiting. */
smp_mb();
smp_mb_placeholder();
if (unlikely(atomic_read(&p_rcu_reader->waiting))) {
atomic_set(&p_rcu_reader->waiting, false);
qemu_event_set(&rcu_gp_event);

17
include/qemu/sys_membarrier.h Normal file
View File

@ -0,0 +1,17 @@
/*
* Process-global memory barriers
*
* Copyright (c) 2018 Red Hat, Inc.
*
* Author: Paolo Bonzini <pbonzini@redhat.com>
*/
#ifndef QEMU_SYS_MEMBARRIER_H
#define QEMU_SYS_MEMBARRIER_H 1
/* Keep it simple, execute a real memory barrier on both sides. */
static inline void smp_mb_global_init(void) {}
#define smp_mb_global() smp_mb()
#define smp_mb_placeholder() smp_mb()
#endif

9
util/rcu.c
View File

@ -93,10 +93,10 @@ static void wait_for_readers(void)
}
/* Here, order the stores to index->waiting before the loads of
* index->ctr. Pairs with smp_mb() in rcu_read_unlock(),
* index->ctr. Pairs with smp_mb_placeholder() in rcu_read_unlock(),
* ensuring that the loads of index->ctr are sequentially consistent.
*/
smp_mb();
smp_mb_global();
QLIST_FOREACH_SAFE(index, &registry, node, tmp) {
if (!rcu_gp_ongoing(&index->ctr)) {
@ -145,9 +145,9 @@ void synchronize_rcu(void)
qemu_mutex_lock(&rcu_sync_lock);
/* Write RCU-protected pointers before reading p_rcu_reader->ctr.
* Pairs with smp_mb() in rcu_read_lock().
* Pairs with smp_mb_placeholder() in rcu_read_lock().
*/
smp_mb();
smp_mb_global();
qemu_mutex_lock(&rcu_registry_lock);
if (!QLIST_EMPTY(&registry)) {
@ -376,6 +376,7 @@ static void rcu_init_child(void)
static void __attribute__((__constructor__)) rcu_init(void)
{
smp_mb_global_init();
#ifdef CONFIG_POSIX
pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_child);
#endif