linux-kernel - Re: [PATCH v2] locking/mutex: remove rcu_read_lock/unlock as we already disabled preemption

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Date:   Wed, 13 Oct 2021 15:54:02 +0800
From:   "Xu, Yanfei" <yanfei.xu@...driver.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, will@...nel.org, longman@...hat.com,
        boqun.feng@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] locking/mutex: remove rcu_read_lock/unlock as we
 already disabled preemption



On 10/12/21 9:15 PM, Peter Zijlstra wrote:
> [Please note: This e-mail is from an EXTERNAL e-mail address]
> 
> On Fri, Oct 08, 2021 at 11:25:18AM +0800, Yanfei Xu wrote:
>> preempt_disable/enable() is equal to RCU read-side crital section,
>> and the mutex lock slowpath disabled the preemption for the optimistic
>> spinning code. Let's remove the rcu_read_lock/unlock for saving some
>> cycles in hot codes.
>>
>> Signed-off-by: Yanfei Xu <yanfei.xu@...driver.com>
>> ---
>> v1->v2: fix the incorrect comment in code and commit message.
>>          thanks for WaiMan's suggestion.
>>
>> BTW, sorry for this late v2 due to a long vocation.
>>
>>   kernel/locking/mutex.c | 10 +++++-----
>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
>> index 2fede72b6af5..2f654cfb10d9 100644
>> --- a/kernel/locking/mutex.c
>> +++ b/kernel/locking/mutex.c
>> @@ -351,13 +351,14 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
>>   {
>>        bool ret = true;
> 
>          lockdep_assert_preemption_disabled();

Agree.

> 
>> -     rcu_read_lock();
>>        while (__mutex_owner(lock) == owner) {
>>                /*
>>                 * Ensure we emit the owner->on_cpu, dereference _after_
> 
> And did you check the other code in locking/ for similar things?
> 

I did a check, rwsem also has the similar things. Will do it for rwsem 
in v3.



Thanks,
Yanfei