linux-kernel - Re: [PATCH] rcuwait: do not enter RCU protection unless a wakeup is needed

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Message-ID: <659fbd82-7d18-0e76-6fe4-b311897b4ae0@redhat.com>
Date:   Wed, 20 Oct 2021 13:37:54 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Oleg Nesterov <oleg@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [PATCH] rcuwait: do not enter RCU protection unless a wakeup is
 needed

On 20/10/21 13:17, Peter Zijlstra wrote:
> AFAICT, rcu_read_lock() for PREEMPT_RCU is:
> 
>    WRITE_ONCE(current->rcu_read_lock_nesting, READ_ONCE(current->rcu_read_lock_nesting) + 1);
>    barrier();

rcu_read_unlock() is the expensive one if you need to go down 
rcu_read_unlock_special().

Paolo

> Paul?