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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH 2/2] locking/rwsem: Use rcu_read_lock_sched to simplify
 codes
To:     Yanfei Xu <yanfei.xu@windriver.com>, peterz@infradead.org,
        mingo@redhat.com, will@kernel.org, boqun.feng@gmail.com
Cc:     linux-kernel@vger.kernel.org
References: <20210926101624.2460704-1-yanfei.xu@windriver.com>
 <20210926101624.2460704-2-yanfei.xu@windriver.com>
Message-ID: <4e4c9adf-5444-e331-fefa-0d72aea8ba57@redhat.com>
Date:   Sun, 26 Sep 2021 15:22:19 -0400
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On 9/26/21 6:16 AM, Yanfei Xu wrote:
> Use rcu_read_lock_sched to simplify the codes, and it also saves
> some cycles of handling rcu nesting counter.
>
> Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
> ---
>   kernel/locking/rwsem.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 000e8d5a2884..7afadfe02286 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -616,8 +616,7 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
>   		return false;
>   	}
>   
> -	preempt_disable();
> -	rcu_read_lock();
> +	rcu_read_lock_sched();
>   	owner = rwsem_owner_flags(sem, &flags);
>   	/*
>   	 * Don't check the read-owner as the entry may be stale.
> @@ -625,8 +624,7 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
>   	if ((flags & RWSEM_NONSPINNABLE) ||
>   	    (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
>   		ret = false;
> -	rcu_read_unlock();
> -	preempt_enable();
> +	rcu_read_unlock_sched();
>   
>   	lockevent_cond_inc(rwsem_opt_fail, !ret);
>   	return ret;

I don't think there is any performance gain with this change. I would 
prefer the original code that is more readable as some people may not 
know rcu_read_lock_sched() will disable preemption if they don't look 
into it.

Cheers,
Longman