Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751914AbbFMGxG (ORCPT ); Sat, 13 Jun 2015 02:53:06 -0400 Received: from mail-wg0-f42.google.com ([74.125.82.42]:34499 "EHLO mail-wg0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751005AbbFMGxA (ORCPT ); Sat, 13 Jun 2015 02:53:00 -0400 Date: Sat, 13 Jun 2015 08:52:55 +0200 From: Ingo Molnar To: Oleg Nesterov Cc: linux-kernel@vger.kernel.org, linux-mml@vger.kernel.org, Andy Lutomirski , Andrew Morton , Denys Vlasenko , Brian Gerst , Peter Zijlstra , Borislav Petkov , "H. Peter Anvin" , Linus Torvalds , Thomas Gleixner , Waiman Long Subject: Re: [PATCH 06/12] x86/mm: Enable and use the arch_pgd_init_late() method Message-ID: <20150613065255.GA16018@gmail.com> References: <1434031637-9091-1-git-send-email-mingo@kernel.org> <1434031637-9091-7-git-send-email-mingo@kernel.org> <20150612225000.GA24699@redhat.com> <20150613064705.GA13835@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150613064705.GA13835@gmail.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2306 Lines: 79 * Ingo Molnar wrote: > * Oleg Nesterov wrote: > > > On 06/11, Ingo Molnar wrote: > > > > > > +void arch_pgd_init_late(struct mm_struct *mm, pgd_t *pgd) > > > +{ > > > + /* > > > + * This is called after a new MM has been made visible > > > + * in fork() or exec(). > > > + * > > > + * This barrier makes sure the MM is visible to new RCU > > > + * walkers before we initialize it, so that we don't miss > > > + * updates: > > > + */ > > > + smp_wmb(); > > > > I can't understand the comment and the barrier... > > > > Afaics, we need to ensure that: > > > > > + if (pgd_val(*pgd_src)) > > > + WRITE_ONCE(*pgd_dst, *pgd_src); > > > > either we notice the recent update of this PGD, or (say) the subsequent > > sync_global_pgds() can miss the child. > > > > How the write barrier can help? > > So the real thing this pairs with is the earlier: > > tsk->mm = mm; > > plus the linking of the new task in the task list. > > _that_ write must become visible to others before we do the (conditional) copy > ourselves. > > Granted, it happens quite a bit earlier, and the task linking's use of locking > is a natural barrier - but since this is lockless I didn't want to leave a > silent assumption in. > > Perhaps remove the barrier and just leave a comment in that describes the > assumption on task-linking being a full barrier? Ah, there's another detail I forgot. This might handle the fork case, but in exec() we have: tsk->mm = mm; arch_pgd_init_late(mm); and since the task is already linked, here we need the barrier. So how about I improve the comment to: /* * This function is called after a new MM has been made visible * in fork() or exec() via: * * tsk->mm = mm; * * This barrier makes sure the MM is visible to new RCU * walkers before we initialize the pagetables below, so that * we don't miss updates: */ smp_wmb(); and leave the barrier there? Thanks, Ingo -- 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/