[RFC PATCH 2/8] rcu/nocb: Prepare to change nocb cpumask from CPU-hotplug protected cpuset caller

Waiman Long
2024-01-17T16:36:15Z

From: Frederic Weisbecker <frederic@kernel.org>

cpusets is going to use the NOCB (de-)offloading interface while
holding hotplug lock. Therefore pull out the responsibility of protecting
against concurrent CPU-hotplug changes to the callers of
rcu_nocb_cpumask_update().

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Zefan Li <lizefan.x@bytedance.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Phil Auld <pauld@redhat.com>
Cc: Nicolas Saenz Julienne <nsaenz@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
---
kernel/rcu/rcutorture.c | 2 ++
kernel/rcu/tree_nocb.h | 4 ++--
2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 228a5488eb5e..e935152346ff 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -2139,6 +2139,7 @@ static int rcu_nocb_toggle(void *arg)
do {
r = torture_random(&rand);
cpu = (r >> 1) % (maxcpu + 1);
+ cpus_read_lock();
if (r & 0x1) {
rcu_nocb_cpumask_update(cpumask_of(cpu), true);
atomic_long_inc(&n_nocb_offload);
@@ -2146,6 +2147,7 @@ static int rcu_nocb_toggle(void *arg)
rcu_nocb_cpumask_update(cpumask_of(cpu), false);
atomic_long_inc(&n_nocb_deoffload);
}
+ cpus_read_unlock();
toggle_delay = torture_random(&rand) % toggle_fuzz + toggle_interval;
set_current_state(TASK_INTERRUPTIBLE);
schedule_hrtimeout(&toggle_delay, HRTIMER_MODE_REL);
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 60b0a15ed6e2..bbcf6f4152a3 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1301,12 +1301,13 @@ int rcu_nocb_cpumask_update(const struct cpumask *cpumask, bool offload)
int err_cpu;
cpumask_var_t saved_nocb_mask;

+ lockdep_assert_cpus_held();
+
if (!alloc_cpumask_var(&saved_nocb_mask, GFP_KERNEL))
return -ENOMEM;

cpumask_copy(saved_nocb_mask, rcu_nocb_mask);

- cpus_read_lock();
mutex_lock(&rcu_state.barrier_mutex);
for_each_cpu(cpu, cpumask) {
if (offload) {
@@ -1340,7 +1341,6 @@ int rcu_nocb_cpumask_update(const struct cpumask *cpumask, bool offload)
}

mutex_unlock(&rcu_state.barrier_mutex);
- cpus_read_unlock();

free_cpumask_var(saved_nocb_mask);

--
2.39.3