From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933167AbcFJNHj (ORCPT ); Fri, 10 Jun 2016 09:07:39 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58889 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932261AbcFJNHh (ORCPT ); Fri, 10 Jun 2016 09:07:37 -0400 Date: Fri, 10 Jun 2016 15:07:33 +0200 From: Oleg Nesterov To: Andrea Parri Cc: Peter Zijlstra , LKML Subject: Re: [PATCH] events/uprobes: move smp_read_barrier_depends() where needed Message-ID: <20160610130733.GB8203@redhat.com> References: <1465496321-10898-1-git-send-email-parri.andrea@gmail.com> <20160610070310.GA6843@redhat.com> <20160610122537.GA7776@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Fri, 10 Jun 2016 13:07:37 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/10, Andrea Parri wrote: > > On Fri, Jun 10, 2016 at 2:26 PM, Oleg Nesterov wrote: > > > On 06/10, Oleg Nesterov wrote: > > > > > > On 06/09, Andrea Parri wrote: > > > > > > > > There is no need to use the barrier if there is no dereference/ > > > > memory access; move it where needed (currently, affecting only > > > > Alpha). > > > > > > OK, although area == NULL is unlikely case, > > > > > > > While touching this, also make the reads _ONCE(). > > > > > > Why? both xol_area/vaddr can't change. > > > > and perhaps it would be better to eliminate this smp_read_barrier_depends() > > altogether, we have a lot of helpers. I mean, > > > > static unsigned long get_trampoline_vaddr(void) > > { > > struct xol_area *area; > > > > area = > > lockless_dereference(current->mm->uprobes_state.xol_area); > > if (area) > > return area->vaddr; > > return -1; > > } > > > > looks a bit more simple/clean. Note also another smp_read_barrier_depends() > > in get_xol_area() which can be changed the same way. > > > > What do you think? > > > > More simply/clean, as you said, maybe; one advantage of keeping > the "raw" smp_read_barrier_depends() in get_trampoline_vaddr() is > that we can avoid it when area is NULL; Do you really think it makes sense to optimize out read_barrier_depends here? It can only be NULL in handle_swbp(), and in this case we are going to do a lot of work, and in particular install this xol vma, > a similar solution is adopt- > ed in kernel/task_work.c:task_work_cancel(). Heh ;) this code was written before we had lockless_dereference(). And I do remember I thought that we need such a helper when read_barrier_depends() was added. Oleg.