From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-5.8 required=5.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 138867D04D for ; Tue, 29 Jan 2019 19:09:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727402AbfA2TJH (ORCPT ); Tue, 29 Jan 2019 14:09:07 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:58176 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726984AbfA2TJH (ORCPT ); Tue, 29 Jan 2019 14:09:07 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=Cua5YyY3UCe/2GZUwqOSzY6Efy7wQkjzkYSxkI+PGIc=; b=gVcqUjmOXq5LVw8P80svC9HYP AV8bRa1e6rYmZwXtE6cNeG0FGcm8XcguwKoNEFRTx7ABc+Nx/PtM4QjuVMLk/RfooU0lRdnkWxOqT QRzoVNadl1qrmH2b/smBvJ9lU7mKAOS0dIfiwUBYj+kivO4cslH6ojOFJH/BPXeYDEcZWq2+4VDuH KymK/JiSENNQxnkaa9PJ5pz11okYs9d5b4ZeGmCvOtzi8CKSuHWlJUnZhwhOn4KQ4rEiylU4FISEn iLyexJFSEeziwNmFVxQ7FBZ122QdaCxlvNN0P6++7mqVuXcvrpnc6QI7RMXH5dLVQs1bJW0S+fMlb WpgokPBRg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1goYkf-0003Mo-DW; Tue, 29 Jan 2019 19:08:53 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 84A3F201EC171; Tue, 29 Jan 2019 20:08:51 +0100 (CET) Date: Tue, 29 Jan 2019 20:08:51 +0100 From: Peter Zijlstra To: Suren Baghdasaryan Cc: Greg Kroah-Hartman , Tejun Heo , lizefan@huawei.com, Johannes Weiner , axboe@kernel.dk, dennis@kernel.org, Dennis Zhou , Ingo Molnar , Andrew Morton , Jonathan Corbet , cgroups@vger.kernel.org, linux-mm , linux-doc@vger.kernel.org, LKML , kernel-team@android.com Subject: Re: [PATCH v3 5/5] psi: introduce psi monitor Message-ID: <20190129190851.GA2961@hirez.programming.kicks-ass.net> References: <20190124211518.244221-1-surenb@google.com> <20190124211518.244221-6-surenb@google.com> <20190129123843.GK28467@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Tue, Jan 29, 2019 at 10:18:20AM -0800, Suren Baghdasaryan wrote: > On Tue, Jan 29, 2019 at 4:38 AM Peter Zijlstra wrote: > > > > On Thu, Jan 24, 2019 at 01:15:18PM -0800, Suren Baghdasaryan wrote: > > > + atomic_set(&group->polling, polling); > > > + /* > > > + * Memory barrier is needed to order group->polling > > > + * write before times[] read in collect_percpu_times() > > > + */ > > > + smp_mb__after_atomic(); > > > > That's broken, smp_mb__{before,after}_atomic() can only be used on > > atomic RmW operations, something atomic_set() is _not_. > > Oh, I didn't realize that. After reading the following example from > atomic_ops.txt That document it woefully out of date (and I should double check, but I think we can actually delete it now). Please see Documentation/atomic_t.txt > I was under impression that smp_mb__after_atomic() > would make changes done by atomic_set() visible: > > /* All memory operations before this call will > * be globally visible before the clear_bit(). > */ > smp_mb__before_atomic(); > clear_bit( ... ); > /* The clear_bit() will be visible before all > * subsequent memory operations. > */ > smp_mb__after_atomic(); > > but I'm probably missing something. Is there a more detailed > description of these rules anywhere else? See atomic_t.txt; but the difference is that clear_bit() is a RmW, while atomic_set() is just a plain store. > Meanwhile I'll change smp_mb__after_atomic() into smp_mb(). Would that > fix the ordering? It would work here; but I'm still trying to actually understand all this. So while the detail would be fine, I'm not ready to judge the over-all thing.