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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id nb29sm1084687ejc.54.2021.10.20.04.37.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 20 Oct 2021 04:37:55 -0700 (PDT) Message-ID: <659fbd82-7d18-0e76-6fe4-b311897b4ae0@redhat.com> Date: Wed, 20 Oct 2021 13:37:54 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.1.0 Subject: Re: [PATCH] rcuwait: do not enter RCU protection unless a wakeup is needed Content-Language: en-US To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Davidlohr Bueso , Oleg Nesterov , Ingo Molnar , "Paul E . McKenney" , Wanpeng Li References: <20211020110638.797389-1-pbonzini@redhat.com> From: Paolo Bonzini In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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?