Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754187AbbFMRqm (ORCPT ); Sat, 13 Jun 2015 13:46:42 -0400 Received: from mx1.redhat.com ([209.132.183.28]:51082 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753937AbbFMRqd (ORCPT ); Sat, 13 Jun 2015 13:46:33 -0400 Date: Sat, 13 Jun 2015 19:45:27 +0200 From: Oleg Nesterov To: Ingo Molnar 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: <20150613174527.GA29379@redhat.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> <20150613065255.GA16018@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150613065255.GA16018@gmail.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1683 Lines: 60 On 06/13, Ingo Molnar wrote: > > * Ingo Molnar wrote: > > > * Oleg Nesterov wrote: > > > > > > > > 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. Hmm. that write must be visible before we start to _read_ *pgd_src, afaics. > > 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. I agree, > 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); Yes, this too. But wmb() can't help. At least we need the full mb() to serialize the STORE above (or list addition in copy_process) with the LOAD which reads *pgd_src. Plus we need another mb() in sync_global_pgds(), say, before the main for_each_process() loop. it would be nice to make this more simple/clear somehow... Oleg. -- 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/