To support onlining multiple CPUs concurrently,
change rcu_state.n_online_cpus updates to be atomic.
Note, it's ok for rcu_blocking_is_gp() to do a
atomic_read(&rcu_state.n_online_cpus), as the
value of .n_online_cpus switches from 1->2, in
rcutree_prepare_cpu(), which runs before the new
CPU comes online. Similarly 2->1 transition happens
from rcutree_dead_cpu(), which executes after the
CPU is offlined, and runs on the last online CPU.
Signed-off-by: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
---
kernel/rcu/tree.c | 6 +++---
kernel/rcu/tree.h | 2 +-
2 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 3da7826865f7..c1db01c4ea39 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2460,7 +2460,7 @@ 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);
+ atomic_dec(&rcu_state.n_online_cpus);
/* Adjust any no-longer-needed kthreads. */
rcu_boost_kthread_setaffinity(rnp, -1);
// Stop-machine done, so allow nohz_full to disable tick.
@@ -3740,7 +3740,7 @@ static int rcu_blocking_is_gp(void)
* 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;
+ ret = atomic_read(&rcu_state.n_online_cpus) <= 1;
preempt_enable();
return ret;
}
@@ -4199,7 +4199,7 @@ int rcutree_prepare_cpu(unsigned int cpu)
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
rcu_spawn_one_boost_kthread(rnp);
rcu_spawn_cpu_nocb_kthread(cpu);
- WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus + 1);
+ atomic_inc(&rcu_state.n_online_cpus);
return 0;
}
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 4b889081f4f4..f1017e7e1e9e 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -299,7 +299,7 @@ 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. */
+ atomic_t n_online_cpus; /* # CPUs online for RCU. */
/* The following fields are guarded by the root rcu_node's lock. */
--
2.17.1
