From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f174.google.com (mail-pd0-f174.google.com [209.85.192.174]) by kanga.kvack.org (Postfix) with ESMTP id 5F9766B0031 for ; Tue, 24 Sep 2013 13:09:15 -0400 (EDT) Received: by mail-pd0-f174.google.com with SMTP id y13so4885583pdi.33 for ; Tue, 24 Sep 2013 10:09:15 -0700 (PDT) Date: Tue, 24 Sep 2013 19:02:22 +0200 From: Oleg Nesterov Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() Message-ID: <20130924170222.GA5059@redhat.com> References: <20130917143003.GA29354@twins.programming.kicks-ass.net> <20130917162050.GK22421@suse.de> <20130917164505.GG12926@twins.programming.kicks-ass.net> <20130918154939.GZ26785@twins.programming.kicks-ass.net> <20130919143241.GB26785@twins.programming.kicks-ass.net> <20130923175052.GA20991@redhat.com> <20130924123821.GT12926@twins.programming.kicks-ass.net> <20130924160359.GA2739@redhat.com> <20130924164900.GG9093@linux.vnet.ibm.com> <20130924165437.GR9326@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130924165437.GR9326@twins.programming.kicks-ass.net> Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: "Paul E. McKenney" , Mel Gorman , Rik van Riel , Srikar Dronamraju , Ingo Molnar , Andrea Arcangeli , Johannes Weiner , Linux-MM , LKML , Thomas Gleixner , Steven Rostedt On 09/24, Peter Zijlstra wrote: > > On Tue, Sep 24, 2013 at 09:49:00AM -0700, Paul E. McKenney wrote: > > > > void cpu_hotplug_done(void) > > > > { > > > > + /* Signal the writer is done */ > > > > + cpuhp_writer = 0; > > > > + wake_up_all(&cpuhp_wq); > > > > + > > > > + /* Wait for any pending readers to be running */ > > > > + cpuhp_writer_wait(!atomic_read(&cpuhp_waitcount)); > > > > + cpuhp_writer_task = NULL; > > > > > > We also need to ensure that the next reader should see all changes > > > done by the writer, iow this lacks "realease" semantics. > > > > Good point -- I was expecting wake_up_all() to provide the release > > semantics, but code could be reordered into __wake_up()'s critical > > section, especially in the case where there was nothing to wake > > up, but where there were new readers starting concurrently with > > cpu_hotplug_done(). > > Doh, indeed. I missed this in Oleg's email, but yes I made that same > assumption about wake_up_all(). Well, I think this is even worse... No matter what the writer does, the new reader needs mb() after it checks !__cpuhp_writer. Or we need another synchronize_sched() in cpu_hotplug_done(). This is what percpu_rw_semaphore() does (to remind, this can be turned into call_rcu). Oleg. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org