Re: [PATCH rcu 2/7] rcu/nocb: Invert rcu_state.barrier_mutex VS hotplug lock locking order

[Neeraj Upadhyay]

On 6/21/2022 4:14 AM, Paul E. McKenney wrote:
> From: Zqiang <qiang1.zhang@xxxxxxxxx>
>
> In case of failure to spawn either rcuog or rcuo[p] kthreads for a given
> rdp, rcu_nocb_rdp_deoffload() needs to be called with the hotplug
> lock and the barrier_mutex held. However cpus write lock is already held
> while calling rcutree_prepare_cpu(). It's not possible to call
> rcu_nocb_rdp_deoffload() from there with just locking the barrier_mutex
> or this would result in a locking inversion against
> rcu_nocb_cpu_deoffload() which holds both locks in the reverse order.
>
> Simply solve this with inverting the locking order inside
> rcu_nocb_cpu_[de]offload(). This will be a pre-requisite to toggle NOCB
> states toward cpusets anyway.
>
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Uladzislau Rezki <uladzislau.rezki@xxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---

Reviewed-by: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>


Thanks
Neeraj

>   kernel/rcu/tree_nocb.h | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index dac74952e1d1b..f2f2cab6285a1 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1055,8 +1055,8 @@ int rcu_nocb_cpu_deoffload(int cpu)
>   	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
>   	int ret = 0;
>   
> -	mutex_lock(&rcu_state.barrier_mutex);
>   	cpus_read_lock();
> +	mutex_lock(&rcu_state.barrier_mutex);
>   	if (rcu_rdp_is_offloaded(rdp)) {
>   		if (cpu_online(cpu)) {
>   			ret = work_on_cpu(cpu, rcu_nocb_rdp_deoffload, rdp);
> @@ -1067,8 +1067,8 @@ int rcu_nocb_cpu_deoffload(int cpu)
>   			ret = -EINVAL;
>   		}
>   	}
> -	cpus_read_unlock();
>   	mutex_unlock(&rcu_state.barrier_mutex);
> +	cpus_read_unlock();
>   
>   	return ret;
>   }
> @@ -1134,8 +1134,8 @@ int rcu_nocb_cpu_offload(int cpu)
>   	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
>   	int ret = 0;
>   
> -	mutex_lock(&rcu_state.barrier_mutex);
>   	cpus_read_lock();
> +	mutex_lock(&rcu_state.barrier_mutex);
>   	if (!rcu_rdp_is_offloaded(rdp)) {
>   		if (cpu_online(cpu)) {
>   			ret = work_on_cpu(cpu, rcu_nocb_rdp_offload, rdp);
> @@ -1146,8 +1146,8 @@ int rcu_nocb_cpu_offload(int cpu)
>   			ret = -EINVAL;
>   		}
>   	}
> -	cpus_read_unlock();
>   	mutex_unlock(&rcu_state.barrier_mutex);
> +	cpus_read_unlock();
>   
>   	return ret;
>   }