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

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Date:   Sun, 26 Sep 2021 20:46:59 -0400
From:   Waiman Long <llong@...hat.com>
To:     Yanfei Xu <yanfei.xu@...driver.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 9/26/21 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.

Cheers,
Longman