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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id z23sm14061545ljz.52.2020.04.16.08.38.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 16 Apr 2020 08:38:50 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Thu, 16 Apr 2020 17:38:44 +0200 To: Sebastian Andrzej Siewior Cc: Uladzislau Rezki , rcu@vger.kernel.org, "Paul E. McKenney" , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Thomas Gleixner , Mike Galbraith Subject: Re: [PATCH 1/3] rcu: Use static initializer for krc.lock Message-ID: <20200416153844.GA8191@pc636> References: <20200415160034.662274-2-bigeasy@linutronix.de> <20200416144254.GC90777@google.com> <20200416150038.GA7772@pc636> <20200416152027.wkpxr2fvuf4jkgct@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200416152027.wkpxr2fvuf4jkgct@linutronix.de> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org Sebastian, > On 2020-04-16 17:01:43 [+0200], Uladzislau Rezki wrote: > > > > @@ -3139,10 +3136,8 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func) > > > > unsigned long flags; > > > > struct kfree_rcu_cpu *krcp; > > > > > > > > - local_irq_save(flags); // For safely calling this_cpu_ptr(). > > > > - krcp = this_cpu_ptr(&krc); > > > > - if (krcp->initialized) > > > > - spin_lock(&krcp->lock); > > > > + krcp = raw_cpu_ptr(&krc); > > > > + spin_lock_irqsave(&krcp->lock, flags); > > > > > It is not a good way to access to per-CPU variable. There is a race in > > your code. So, we will rework it anyway soon. To guarantee that we stay > > on the same CPU, first we disable IRQ's then we access per-CPU var and > > take a spinlock. > > What is the worst thing that can happen if a CPU migration happens > between raw_cpu_ptr() and spin_lock_irqsave() ? > You will just introduce higher rate of lock contention what we would like to get rid of. Could you please explain what is broken when you do like: local_irq_save(flags); krcp = this_cpu_ptr(&krc); spin_lock(&krcp->lock); Thanks! -- Vlad Rezki