Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755789AbcDGJrF (ORCPT ); Thu, 7 Apr 2016 05:47:05 -0400 Received: from mx2.suse.de ([195.135.220.15]:57526 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755367AbcDGJrC (ORCPT ); Thu, 7 Apr 2016 05:47:02 -0400 Date: Thu, 7 Apr 2016 11:47:00 +0200 From: Petr Mladek To: Josh Poimboeuf Cc: Jiri Kosina , Jessica Yu , Miroslav Benes , linux-kernel@vger.kernel.org, live-patching@vger.kernel.org, Vojtech Pavlik Subject: Re: [RFC PATCH v1.9 05/14] sched: horrible way to detect whether a task has been preempted Message-ID: <20160407094700.GA27670@pathway.suse.cz> References: <24db5a6ae5b63dfcd2096a12d18e1399a351348e.1458933243.git.jpoimboe@redhat.com> <20160406130619.GA5218@pathway.suse.cz> <20160406163356.hba6jzkloaukknn4@treble.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160406163356.hba6jzkloaukknn4@treble.redhat.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 Content-Length: 1467 Lines: 46 On Wed 2016-04-06 11:33:56, Josh Poimboeuf wrote: > On Wed, Apr 06, 2016 at 03:06:19PM +0200, Petr Mladek wrote: > > On Fri 2016-03-25 14:34:52, Josh Poimboeuf wrote: > > > This is a horrible way to detect whether a task has been preempted. > > > Come up with something better: task flag? or is there already an > > > existing mechanism? > > > > What about using kallsyms_lookup_size_offset() to check the address. > > It is more heavyweight but less hacky. The following code seems > > to work for me: > > > > bool in_preempt_schedule_irq(unsigned long addr) > > { > > static unsigned long size; > > > > if (unlikely(!size)) { > > int ret; > > > > ret = kallsyms_lookup_size_offset( > > (unsigned long)preempt_schedule_irq, > > size, NULL); ^^^^ It works even better with &size ;-) > > > > /* > > * Warn when the function is used without kallsyms or > > * when it is unable to locate preempt_schedule_irq(). > > * Be conservative and always return true in this case. > > */ > > if (WARN_ON(!ret)) > > size = -1L; > > } > > > > return (addr - (unsigned long)preempt_schedule_irq <= size); > > } > > Yeah, that would definitely be better. Though still somewhat hacky. Yeah. Well this is the same approach that we use to check if a patched function is on the stack. We could even move this check into the livepatch code but then print_context_stack_reliable() will not always give reliable results. Best Regards, Petr