[PATCH] rcu/tasks: Use static per cpu work_struct for Tasks Rude grace periods

[PATCH] rcu/tasks: Use static per cpu work_struct for Tasks Rude grace periods

[Bradley Morgan]

Zqiang caught a snafu in the rude grace period, if a too small grace
period was reached, a UAF could occur. However, his patch used
a WARN_ONCE() on schedule_on_each_cpu() OOM failures, which would not
have handled the failure properly.

Instead of throwing a warning or relying on dynamic alloc_percpu(), replace
the dynamic allocation with static per cpu work_struct instances. Using
schedule_work_on() and flush_work() keeps everything in task context,
guarantees the required context switch on every online CPU, and stays
zero allocation so it cannot fail under OOM.

Suggested-by: Zqiang <qiang.zhang@linux.dev>
Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 kernel/rcu/tasks.h | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 627295396cd9..f81c0ea0b92a 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1348,11 +1348,25 @@ static void rcu_tasks_be_rude(struct work_struct *work)
 {
 }
 
+static DEFINE_PER_CPU(struct work_struct, tasks_rude_work);
+
 // Wait for one rude RCU-tasks grace period.
 static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
 {
+	int cpu;
+
+	guard(cpus_read_lock)();
 	rtp->n_ipis += cpumask_weight(cpu_online_mask);
-	schedule_on_each_cpu(rcu_tasks_be_rude);
+
+	for_each_online_cpu(cpu) {
+		struct work_struct *work = &per_cpu(tasks_rude_work, cpu);
+
+		INIT_WORK(work, rcu_tasks_be_rude);
+		schedule_work_on(cpu, work);
+	}
+
+	for_each_online_cpu(cpu)
+		flush_work(&per_cpu(tasks_rude_work, cpu));
 }
 
 static void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
-- 
2.53.0

Re: [PATCH] rcu/tasks: Use static per cpu work_struct for Tasks Rude grace periods

[Paul E. McKenney]

On Sat, Oct 17, 2026 at 12:33:13PM +0000, Bradley Morgan wrote:
> Zqiang caught a snafu in the rude grace period, if a too small grace
> period was reached, a UAF could occur. However, his patch used
> a WARN_ONCE() on schedule_on_each_cpu() OOM failures, which would not
> have handled the failure properly.
> 
> Instead of throwing a warning or relying on dynamic alloc_percpu(), replace
> the dynamic allocation with static per cpu work_struct instances. Using
> schedule_work_on() and flush_work() keeps everything in task context,
> guarantees the required context switch on every online CPU, and stays
> zero allocation so it cannot fail under OOM.
> 
> Suggested-by: Zqiang <qiang.zhang@linux.dev>
> Signed-off-by: Bradley Morgan <brads@mainlining.org>

Sorry, Bradley, but no, absolutely not.

Because this not taking a suggestion, but is instead *exactly* Zqiang's
patch.  We do not commit plagiarism in the Linux-kernel community.

Zqiang, please submit a proper patch with appropriate credit to Bradley.
The obvious approach would be to following his example, that is, to
credit him using Suggested-by, but Reported-by or a sentence calling out
his feedback in the commit log would suffice.  However, I will defer to
your best judgment.

							Thanx, Paul

> ---
>  kernel/rcu/tasks.h | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index 627295396cd9..f81c0ea0b92a 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -1348,11 +1348,25 @@ static void rcu_tasks_be_rude(struct work_struct *work)
>  {
>  }
>  
> +static DEFINE_PER_CPU(struct work_struct, tasks_rude_work);
> +
>  // Wait for one rude RCU-tasks grace period.
>  static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
>  {
> +	int cpu;
> +
> +	guard(cpus_read_lock)();
>  	rtp->n_ipis += cpumask_weight(cpu_online_mask);
> -	schedule_on_each_cpu(rcu_tasks_be_rude);
> +
> +	for_each_online_cpu(cpu) {
> +		struct work_struct *work = &per_cpu(tasks_rude_work, cpu);
> +
> +		INIT_WORK(work, rcu_tasks_be_rude);
> +		schedule_work_on(cpu, work);
> +	}
> +
> +	for_each_online_cpu(cpu)
> +		flush_work(&per_cpu(tasks_rude_work, cpu));
>  }
>  
>  static void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
> -- 
> 2.53.0