From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753398AbdJRLZQ (ORCPT ); Wed, 18 Oct 2017 07:25:16 -0400 Received: from mx2.suse.de ([195.135.220.15]:51475 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753337AbdJRLZN (ORCPT ); Wed, 18 Oct 2017 07:25:13 -0400 Date: Wed, 18 Oct 2017 13:25:10 +0200 From: Petr Mladek To: Miroslav Benes Cc: Jason Baron , Josh Poimboeuf , linux-kernel@vger.kernel.org, live-patching@vger.kernel.org, jeyu@kernel.org, jikos@kernel.org Subject: Re: [PATCH v3 2/2] livepatch: add atomic replace Message-ID: <20171018112510.GU2795@pathway.suse.cz> References: <20171006223247.fkkrrtxydq33c7dw@treble> <3b0c1c00-e4ae-13ab-434d-73d8a5e5a60d@akamai.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 2017-10-18 11:10:09, Miroslav Benes wrote: > On Tue, 17 Oct 2017, Jason Baron wrote: > > If the atomic replace patch does > > not contain any immediates, then we can drop the reference on the > > immediately preceding patch only. That is because there may have been > > previous transitions to immediate functions in the func stack, and the > > transition to the atomic replace patch only checks immediately preceding > > transition. It would be possible to check all of the previous immediate > > function transitions, but this adds complexity and seems like not a > > common pattern. So I would suggest that we just drop the reference on > > the previous patch if the atomic replace patch does not contain any > > immediate functions. > > It is even more complicated and it is not connected only to atomic replace > patch (I realized this while reading the first part of your email and > then you confirmed it with this paragraph). The consistency model is > broken with respect to immediate patches. > > func a > patches 1i > 2i > 3 > > Now, when you're applying 3, only 2i function is checked. But there might > be a task sleeping in 1i. Such task would be migrated to 3, because we do > not check 1 in klp_check_stack_func() at all. > > I see three solutions. > > 1. Say it is an user's fault. Since it is not obvious and it is > easy-to-make mistake, I would not go this way. > > 2. We can fix klp_check_stack_func() in an exact way you're proposing. > We'd go back in func stack as long as there are immediate patches there. > This adds complexity and I'm not sure if all the problems would be solved > because scenarios how patches are stacked and applied to different > functions may be quite complex. > > 3. Drop immediate. It causes problems only and its advantages on x86_64 > are theoretical. You would still need to solve the interaction with atomic > replace on other architecture with immediate preserved, but that may be > easier. Or we can be aggressive and drop immediate completely. The force > transition I proposed earlier could achieve the same. To make it clear. We currently rely on the immediate handling on architectures without a reliable stack checking. The question is if anyone uses it for another purpose in practice. A solution would be to remove the per-func immediate flag and invert the logic of the per-patch one. We could rename it to something like "consistency_required" or "semantic_changes". A patch with this flag set then might be refused on systems without reliable stacks. Otherwise, the consistency model would be used for all patches. As a result, all patches would be handled either using the consistency model or immediately. We would need to care about any mix of these. Best Regards, Petr