From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756986AbcJYOdR (ORCPT ); Tue, 25 Oct 2016 10:33:17 -0400 Received: from foss.arm.com ([217.140.101.70]:48338 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755401AbcJYOdK (ORCPT ); Tue, 25 Oct 2016 10:33:10 -0400 Date: Tue, 25 Oct 2016 15:32:37 +0100 From: Mark Rutland To: Joe Perches Cc: linux-kernel@vger.kernel.org, Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] arm64: Neaten show_regs, remove KERN_CONT Message-ID: <20161025143237.GD8898@leverpostej> References: <4cbf196b83cd9d175634e7056744dc649ae87f63.1477253239.git.joe@perches.com> <20161024113131.GH15620@leverpostej> <1477326477.1984.2.camel@perches.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1477326477.1984.2.camel@perches.com> 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 Mon, Oct 24, 2016 at 09:27:57AM -0700, Joe Perches wrote: > On Mon, 2016-10-24 at 12:31 +0100, Mark Rutland wrote: > > On Sun, Oct 23, 2016 at 01:40:49PM -0700, Joe Perches wrote: > > > commit db4b0710fae9 ("arm64: fix show_regs fallout from KERN_CONT changes") > > > corrected the KERN_CONT fallout from commit 4bcc595ccd80 > > > ("printk: reinstate KERN_CONT for printing continuation lines"), but > > > the code still has unnecessary KERN_CONT uses. Remove them. > > > > Why are these unnecessary KERN_CONTs a larger problem than duplicating > > the format string for a third time? Having to duplicate it at all was > > annoying enough. > > Not printing partial lines is the best solution to avoiding > message output interleaving. Looking further, it seems that KERN_CONT is terminally broken. The core code somehow swallows newlines from some KERN_CONT prints in a non-deterministic fashion, and also appears to insert newlines from thin air. This happens in the absence of intervening printks. With the current code in v4.9-rc2, we get output like: x29: 0000ffffe4938c80 x28: 0000000000000000 x27: 0000000000000000 x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000 x23: 0000000000000000 x22: 0000000000000000 x21: 0000000000400470 x20: 0000000000000000 x19: 0000000000000000 x18: 0000ffffe4938b60 x17: 0000000000411000 x16: 0000ffff82f72c9c x15: 0000ffff830c8000 x14: 0000000000000040 x13: 0000ffff830c8028 x12: 0000000000008738 x11: 0000000000000008 x10: 00000000ffffffff x9 : 0000ffff830b4e40 x8 : 2f2f2f2f2f2f2f2f x7 : b3b3bab7acff8b8a x6 : 0000ffff83097aa8 x5 : 54d58839205d3679 x4 : 0000000000000000 x3 : 00000000004005d0 x2 : ffff000000000000 x1 : 0000ffffe4938e08 x0 : ffff000000000000 ... or: x29: 0000fffff6f6a600 x28: 0000000000000000 x27: 0000000000000000 x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000 x23: 0000000000000000 x22: 0000000000000000 x21: 0000000000400470 x20: 0000000000000000 x19: 0000000000000000 x18: 0000fffff6f6a4e0 x17: 0000000000411000 x16: 0000ffffa6e1fc9c x15: 0000ffffa6f75000 x14: 0000000000000040 x13: 0000ffffa6f75028 x12: 0000000000008738 x11: 0000000000000008 x10: 00000000ffffffff x9 : 0000ffffa6f61e40 x8 : 2f2f2f2f2f2f2f2f x7 : b3b3bab7acff8b8a x6 : 0000ffffa6f44aa8 x5 : 874b6ebb9d5e2f3d x4 : 0000000000000000 x3 : 00000000004005d0 x2 : ffff000000000000 x1 : 0000fffff6f6a788 x0 : ffff000000000000 ... and of course, the buffer shown by $(dmesg) or $(demsg -T) is equally insane, but different. I found that adding a space prior to newlines prevented them from being swallowed, but $(dmesg) would still suffer from random additions. Given all that, unless the core code is changed to as to behave deterministically at least for trivial cases like this one, I think we should avoid KERN_CONT like the plague. So FWIW, so long as you fold in the changes I requested in my other reply, please add: Acked-by: Mark Rutland ... I'll go fix up show_pte() without pr_cont(). Thanks, Mark.