Commit 77a8b846 authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

rcu: make memory barriers more explicit 



Prepare for introducing smp_mb_placeholder() and smp_mb_global().
The new smp_mb() in synchronize_rcu() is not strictly necessary, since
the first atomic_mb_set for rcu_gp_ctr provides the required ordering.
However, synchronize_rcu is not performance critical, and it *will* be
necessary to introduce a smp_mb_global before calling wait_for_readers().

Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 729c0ddd

include/qemu/rcu.h

+13 −2
Original line number Diff line number Diff line
@@ -79,7 +79,10 @@ static inline void rcu_read_lock(void) 
    }



    ctr = atomic_read(&rcu_gp_ctr);

    atomic_xchg(&p_rcu_reader->ctr, ctr);

    atomic_set(&p_rcu_reader->ctr, ctr);



    /* Write p_rcu_reader->ctr before reading RCU-protected pointers.  */

    smp_mb();

}



static inline void rcu_read_unlock(void)
@@ -91,7 +94,15 @@ static inline void rcu_read_unlock(void) 
        return;

    }



    atomic_xchg(&p_rcu_reader->ctr, 0);

    /* 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

     * are sequentially consistent.

     */

    atomic_store_release(&p_rcu_reader->ctr, 0);



    /* Write p_rcu_reader->ctr before reading p_rcu_reader->waiting.  */

    smp_mb();

    if (unlikely(atomic_read(&p_rcu_reader->waiting))) {

        atomic_set(&p_rcu_reader->waiting, false);

        qemu_event_set(&rcu_gp_event);

util/rcu.c

+9 −3
Original line number Diff line number Diff line
@@ -92,8 +92,9 @@ static void wait_for_readers(void) 
            atomic_set(&index->waiting, true);

        }



        /* Here, order the stores to index->waiting before the

         * loads of index->ctr.

        /* Here, order the stores to index->waiting before the loads of

         * index->ctr.  Pairs with smp_mb() in rcu_read_unlock(),

         * ensuring that the loads of index->ctr are sequentially consistent.

         */

        smp_mb();
@@ -142,8 +143,13 @@ static void wait_for_readers(void) 
void synchronize_rcu(void)

{

    qemu_mutex_lock(&rcu_sync_lock);

    qemu_mutex_lock(&rcu_registry_lock);



    /* Write RCU-protected pointers before reading p_rcu_reader->ctr.

     * Pairs with smp_mb() in rcu_read_lock().

     */

    smp_mb();



    qemu_mutex_lock(&rcu_registry_lock);

    if (!QLIST_EMPTY(&registry)) {

        /* In either case, the atomic_mb_set below blocks stores that free

         * old RCU-protected pointers.