linux-kernel - Re: [PATCH 2/2] locking/rwsem: Use rcu_read_lock

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Date:   Tue, 28 Sep 2021 00:41:14 +0800
From:   "Xu, Yanfei" <yanfei.xu@...driver.com>
To:     Waiman Long <llong@...hat.com>, peterz@...radead.org,
        mingo@...hat.com, will@...nel.org, boqun.feng@...il.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] locking/rwsem: Use rcu_read_lock_sched to simplify
 codes



On 9/27/21 3:22 AM, Waiman Long wrote:
> [Please note: This e-mail is from an EXTERNAL e-mail address]
> 
> 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@...driver.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.
> 

OK, thanks for comments.

Yanfei

> Cheers,
> Longman
>