Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762680AbXJRIXK (ORCPT ); Thu, 18 Oct 2007 04:23:10 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1760063AbXJRIWc (ORCPT ); Thu, 18 Oct 2007 04:22:32 -0400 Received: from sceptre.pobox.com ([207.106.133.20]:46343 "EHLO sceptre.pobox.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757407AbXJRIW3 (ORCPT ); Thu, 18 Oct 2007 04:22:29 -0400 Date: Thu, 18 Oct 2007 03:22:21 -0500 From: Nathan Lynch To: Gautham R Shenoy Cc: Linus Torvalds , Andrew Morton , linux-kernel@vger.kernel.org, Srivatsa Vaddagiri , Rusty Russel , Dipankar Sarma , Oleg Nesterov , Ingo Molnar , Paul E McKenney Subject: Re: [RFC PATCH 2/4] Rename lock_cpu_hotplug to get_online_cpus Message-ID: <20071018082221.GE6773@localdomain> References: <20071016103308.GA9907@in.ibm.com> <20071016103506.GB16570@in.ibm.com> <20071017161308.GD6773@localdomain> <20071018075702.GB15281@in.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20071018075702.GB15281@in.ibm.com> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1819 Lines: 45 Gautham R Shenoy wrote: > Hi Nathan, > > Hi Gautham- > > > > Gautham R Shenoy wrote: > > > Replace all lock_cpu_hotplug/unlock_cpu_hotplug from the kernel and use > > > get_online_cpus and put_online_cpus instead as it highlights > > > the refcount semantics in these operations. > > > > Something other than "get_online_cpus", please? lock_cpu_hotplug() > > protects cpu_present_map as well as cpu_online_map. For example, some > > of the powerpc code modified in this patch is made a bit less clear > > because it is manipulating cpu_present_map, not cpu_online_map. > > A quick look at the code, and I am wondering why is lock_cpu_hotplug() > used there in the first place. It doesn't look like we require any > protection against cpus coming up/ going down in the code below, > since the cpu-hotplug operation doesn't affect the cpu_present_map. The locking is necessary. Changes to cpu_online_map and cpu_present_map must be serialized; otherwise you could end up trying to online a cpu as it is being removed (i.e. cleared from cpu_present_map). Online operations must check that a cpu is present before bringing it up (kernel/cpu.c): /* Requires cpu_add_remove_lock to be held */ static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen) { int ret, nr_calls = 0; void *hcpu = (void *)(long)cpu; unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0; if (cpu_online(cpu) || !cpu_present(cpu)) return -EINVAL; .... > Can't we use another mutex here instead of the cpu_hotplug mutex here ? I guess so, but I don't really see the need... - 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/