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rcu: Mark loads from rcu_state.n_online_cpus

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The rcu_state.n_online_cpus value is only ever updated by CPU-hotplug
operations, which are serialized.  However, this value is read locklessly.
This commit therefore marks those reads.  While in the area, it also
adds ASSERT_EXCLUSIVE_WRITER() calls just in case parallel CPU hotplug
becomes a thing.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
This commit is contained in:
Paul E. McKenney 2024-03-07 20:14:55 -08:00 committed by Uladzislau Rezki (Sony)
parent a542d116ba
commit 09e077cf22
2 changed files with 7 additions and 3 deletions
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4
kernel/rcu/tree.c
View File

@ -4328,7 +4328,7 @@ EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);
// whether spinlocks may be acquired safely.
static bool rcu_init_invoked(void)
{
return !!rcu_state.n_online_cpus;
return !!READ_ONCE(rcu_state.n_online_cpus);
}
/*
@ -4538,6 +4538,7 @@ int rcutree_prepare_cpu(unsigned int cpu)
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
rcu_spawn_rnp_kthreads(rnp);
rcu_spawn_cpu_nocb_kthread(cpu);
ASSERT_EXCLUSIVE_WRITER(rcu_state.n_online_cpus);
WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus + 1);
return 0;
@ -4806,6 +4807,7 @@ void rcutree_migrate_callbacks(int cpu)
*/
int rcutree_dead_cpu(unsigned int cpu)
{
ASSERT_EXCLUSIVE_WRITER(rcu_state.n_online_cpus);
WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
// Stop-machine done, so allow nohz_full to disable tick.
tick_dep_clear(TICK_DEP_BIT_RCU);

6
kernel/rcu/tree_stall.h
View File

@ -628,7 +628,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
totqlen += rcu_get_n_cbs_cpu(cpu);
pr_err("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu ncpus=%d)\n",
smp_processor_id(), (long)(jiffies - gps),
(long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus);
(long)rcu_seq_current(&rcu_state.gp_seq), totqlen,
data_race(rcu_state.n_online_cpus)); // Diagnostic read
if (ndetected) {
rcu_dump_cpu_stacks();
@ -689,7 +690,8 @@ static void print_cpu_stall(unsigned long gps)
totqlen += rcu_get_n_cbs_cpu(cpu);
pr_err("\t(t=%lu jiffies g=%ld q=%lu ncpus=%d)\n",
jiffies - gps,
(long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus);
(long)rcu_seq_current(&rcu_state.gp_seq), totqlen,
data_race(rcu_state.n_online_cpus)); // Diagnostic read
rcu_check_gp_kthread_expired_fqs_timer();
rcu_check_gp_kthread_starvation();