diff options
| -rw-r--r-- | include/qemu/rcu.h | 5 | ||||
| -rw-r--r-- | util/rcu.c | 24 |
2 files changed, 15 insertions, 14 deletions
diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h index 313fc41..661c1a1 100644 --- a/include/qemu/rcu.h +++ b/include/qemu/rcu.h @@ -87,7 +87,10 @@ static inline void rcu_read_lock(void) ctr = qatomic_read(&rcu_gp_ctr); qatomic_set(&p_rcu_reader->ctr, ctr); - /* Write p_rcu_reader->ctr before reading RCU-protected pointers. */ + /* + * Read rcu_gp_ptr and write p_rcu_reader->ctr before reading + * RCU-protected pointers. + */ smp_mb_placeholder(); } @@ -83,12 +83,6 @@ static void wait_for_readers(void) */ qemu_event_reset(&rcu_gp_event); - /* Instead of using qatomic_mb_set for index->waiting, and - * qatomic_mb_read for index->ctr, memory barriers are placed - * manually since writes to different threads are independent. - * qemu_event_reset has acquire semantics, so no memory barrier - * is needed here. - */ QLIST_FOREACH(index, ®istry, node) { qatomic_set(&index->waiting, true); } @@ -96,6 +90,10 @@ static void wait_for_readers(void) /* Here, order the stores to index->waiting before the loads of * index->ctr. Pairs with smp_mb_placeholder() in rcu_read_unlock(), * ensuring that the loads of index->ctr are sequentially consistent. + * + * If this is the last iteration, this barrier also prevents + * frees from seeping upwards, and orders the two wait phases + * on architectures with 32-bit longs; see synchronize_rcu(). */ smp_mb_global(); @@ -104,7 +102,7 @@ static void wait_for_readers(void) QLIST_REMOVE(index, node); QLIST_INSERT_HEAD(&qsreaders, index, node); - /* No need for mb_set here, worst of all we + /* No need for memory barriers here, worst of all we * get some extra futex wakeups. */ qatomic_set(&index->waiting, false); @@ -149,26 +147,26 @@ void synchronize_rcu(void) /* Write RCU-protected pointers before reading p_rcu_reader->ctr. * Pairs with smp_mb_placeholder() in rcu_read_lock(). + * + * Also orders write to RCU-protected pointers before + * write to rcu_gp_ctr. */ smp_mb_global(); QEMU_LOCK_GUARD(&rcu_registry_lock); if (!QLIST_EMPTY(®istry)) { - /* In either case, the qatomic_mb_set below blocks stores that free - * old RCU-protected pointers. - */ if (sizeof(rcu_gp_ctr) < 8) { /* For architectures with 32-bit longs, a two-subphases algorithm * ensures we do not encounter overflow bugs. * * Switch parity: 0 -> 1, 1 -> 0. */ - qatomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR); + qatomic_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR); wait_for_readers(); - qatomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR); + qatomic_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR); } else { /* Increment current grace period. */ - qatomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR); + qatomic_set(&rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR); } wait_for_readers(); |