Commit c8d3877e authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

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: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 77a8b846

include/qemu/rcu.h

+4 −3
Original line number Diff line number Diff line
@@ -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);

include/qemu/sys_membarrier.h

0 → 100644
+17 −0
Original line number Diff line number Diff line 
/*

 * 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

util/rcu.c

+5 −4
Original line number Diff line number Diff line
@@ -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