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Date:   Tue, 19 Jul 2022 14:58:04 +0530
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Subject: Re: [PATCH rcu 2/7] rcu/nocb: Invert rcu_state.barrier_mutex VS
 hotplug lock locking order
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To:     "Paul E. McKenney" <paulmck@kernel.org>, <rcu@vger.kernel.org>
CC:     <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>,
        <rostedt@goodmis.org>, Zqiang <qiang1.zhang@intel.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Uladzislau Rezki <uladzislau.rezki@sony.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Frederic Weisbecker <frederic@kernel.org>
References: <20220620224455.GA3840881@paulmck-ThinkPad-P17-Gen-1>
 <20220620224503.3841196-2-paulmck@kernel.org>
From:   Neeraj Upadhyay <quic_neeraju@quicinc.com>
In-Reply-To: <20220620224503.3841196-2-paulmck@kernel.org>
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On 6/21/2022 4:14 AM, Paul E. McKenney wrote:
> From: Zqiang <qiang1.zhang@intel.com>
> 
> 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@intel.com>
> Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Uladzislau Rezki <uladzislau.rezki@sony.com>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---

Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>


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;
>   }