Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754061Ab3IIN3j (ORCPT ); Mon, 9 Sep 2013 09:29:39 -0400 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.122]:23529 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753104Ab3IIN3T (ORCPT ); Mon, 9 Sep 2013 09:29:19 -0400 X-Authority-Analysis: v=2.0 cv=V4T/IJbi c=1 sm=0 a=Sro2XwOs0tJUSHxCKfOySw==:17 a=Drc5e87SC40A:10 a=JDfofD-L5ZgA:10 a=5SG0PmZfjMsA:10 a=kj9zAlcOel0A:10 a=meVymXHHAAAA:8 a=KGjhK52YXX0A:10 a=5p3hctMY2UsA:10 a=4qqQLd7zNwFsm2Afq0sA:9 a=CjuIK1q_8ugA:10 a=jeBq3FmKZ4MA:10 a=Sro2XwOs0tJUSHxCKfOySw==:117 X-Cloudmark-Score: 0 X-Authenticated-User: X-Originating-IP: 67.255.60.225 Date: Mon, 9 Sep 2013 09:29:17 -0400 From: Steven Rostedt To: Steven Rostedt Cc: Frederic Weisbecker , Peter Zijlstra , "Paul E. McKenney" , Eric Dumazet , linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, sbw@mit.edu Subject: Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section? Message-ID: <20130909092917.0c99b71a@gandalf.local.home> In-Reply-To: <20130909092142.05780991@gandalf.local.home> References: <20130906105934.GF20519@somewhere> <20130906151851.GQ3966@linux.vnet.ibm.com> <1378488088.31445.39.camel@edumazet-glaptop> <20130906174117.GU3966@linux.vnet.ibm.com> <20130906185927.GE2706@somewhere> <20130909105347.GK31370@twins.programming.kicks-ass.net> <20130909121329.GA16280@somewhere> <20130909083926.3eceebef@gandalf.local.home> <20130909124547.GB16280@somewhere> <20130909085504.2ddd7e69@gandalf.local.home> <20130909130851.GC16280@somewhere> <20130909092142.05780991@gandalf.local.home> X-Mailer: Claws Mail 3.9.2 (GTK+ 2.24.20; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1646 Lines: 38 On Mon, 9 Sep 2013 09:21:42 -0400 Steven Rostedt wrote: > Especially since the function name itself is "rcu_is_cpu_idle()" which > tells you it's a cpu state, and not a task state. Why would you care in > RCU if CPU 1 is idle if you are on CPU 2? The name should be changed. > Actually, preempt_count is more a CPU state than a task state. When > preempt_count is set, that CPU can not schedule out the current task. > It really has nothing to do with the task itself. It's the CPU that > can't do something. Preempt count should never traverse with a task > from one CPU to another. I'll take this a step further. Here's a simple rule to determine if something is a task state or a CPU state. If the state migrates with a task from one CPU to another, it's a task state. If the state never leaves a CPU with a task, then it's a CPU state. According to the above rules, rcu_is_cpu_idle() is a task state, and really should be in task_struct, and preempt_count is a CPU state, and should be a per_cpu variable. I think the reason preempt_count never was a per cpu variable, is that having it in the stack (thread info) made it easier to test in assembly than having to grab the per cpu info. But I believe it's easier to grab per cpu info in assembly today than it once was, which is why there is a push to move preempt_count to per_cpu where it truly belongs. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/