diff options
Diffstat (limited to 'kernel/rcu')
| -rw-r--r-- | kernel/rcu/tree.c | 19 | ||||
| -rw-r--r-- | kernel/rcu/tree.h | 1 |
2 files changed, 18 insertions, 2 deletions
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 0ccdca441ddf..39e14cf6a9c0 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2396,6 +2396,7 @@ int rcutree_dead_cpu(unsigned int cpu) if (!IS_ENABLED(CONFIG_HOTPLUG_CPU)) return 0; + WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1); /* Adjust any no-longer-needed kthreads. */ rcu_boost_kthread_setaffinity(rnp, -1); /* Do any needed no-CB deferred wakeups from this CPU. */ @@ -3577,7 +3578,20 @@ static int rcu_blocking_is_gp(void) return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE; might_sleep(); /* Check for RCU read-side critical section. */ preempt_disable(); - ret = num_online_cpus() <= 1; + /* + * If the rcu_state.n_online_cpus counter is equal to one, + * there is only one CPU, and that CPU sees all prior accesses + * made by any CPU that was online at the time of its access. + * Furthermore, if this counter is equal to one, its value cannot + * change until after the preempt_enable() below. + * + * Furthermore, if rcu_state.n_online_cpus is equal to one here, + * all later CPUs (both this one and any that come online later + * on) are guaranteed to see all accesses prior to this point + * in the code, without the need for additional memory barriers. + * Those memory barriers are provided by CPU-hotplug code. + */ + ret = READ_ONCE(rcu_state.n_online_cpus) <= 1; preempt_enable(); return ret; } @@ -3622,7 +3636,7 @@ void synchronize_rcu(void) lock_is_held(&rcu_sched_lock_map), "Illegal synchronize_rcu() in RCU read-side critical section"); if (rcu_blocking_is_gp()) - return; + return; // Context allows vacuous grace periods. if (rcu_gp_is_expedited()) synchronize_rcu_expedited(); else @@ -3962,6 +3976,7 @@ int rcutree_prepare_cpu(unsigned int cpu) raw_spin_unlock_irqrestore_rcu_node(rnp, flags); rcu_prepare_kthreads(cpu); rcu_spawn_cpu_nocb_kthread(cpu); + WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus + 1); return 0; } diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index e4f66b8f7c47..805c9eb6f7ae 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -298,6 +298,7 @@ struct rcu_state { /* Hierarchy levels (+1 to */ /* shut bogus gcc warning) */ int ncpus; /* # CPUs seen so far. */ + int n_online_cpus; /* # CPUs online for RCU. */ /* The following fields are guarded by the root rcu_node's lock. */ |