linux-kernel - Re: [PATCH 1/2] locking/mutex: remove rcu_read_lock/unlock as we already disabled preemption

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Date:   Mon, 27 Sep 2021 17:28:47 +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 1/2] locking/mutex: remove rcu_read_lock/unlock as we
 already disabled preemption



On 9/27/21 8:46 AM, Waiman Long wrote:
> [Please note: This e-mail is from an EXTERNAL e-mail address]
> 
> On 9/26/21 3:16 PM, Waiman Long wrote:
>> On 9/26/21 6:16 AM, Yanfei Xu wrote:
>>> preempt_disable/enable() is equal to RCU read-side crital section,
>>> and the mutex lock slowpath disable the preemption throughout the
>>> entire slowpath. Let's remove the rcu_read_lock/unlock for saving
>>> some cycles in hot codes.
>>
>> The description is wrong. Preemption is disabled only in the
>> optimistic spinning code which is not the complete slowpath. Even
>> though it may sound reasonable that disable preemption is likely to
>> prevent reaching quiescent state, but I am not totally sure that will
>> always be the case as there are different RCU favors.
> 
> It does look like that disable preemption can serve as a substitute for
> rcu_read_lock(). However, I will suggest that you also insert a comment
> saying so and the task structure won't go away during the spinning period.
> 

Will send v2, thanks.

Yanfei

> Cheers,
> Longman
>