From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DE5D2C433E6 for ; Thu, 7 Jan 2021 17:37:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9B43F23406 for ; Thu, 7 Jan 2021 17:37:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728796AbhAGRhh (ORCPT ); Thu, 7 Jan 2021 12:37:37 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:46981 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726427AbhAGRhh (ORCPT ); Thu, 7 Jan 2021 12:37:37 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1610040971; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=HAodYt2HQFDdAV3Nlwj/zRiTtI08o4QNW6FChKlHIwI=; b=VREKnqPw8DtlosGR8JZEVxU6wAKZxVSXxfZ2aUggMax4F9+FFmQlL8MJIZgfgpWAtwQ/ib 4JyuQVF8kNVvDJf5rb+mXKRERi75kY5irEdIZRwIHVLKWbLr12Xxk70y/KIW0MnBvUUI/M 8E1i4HnU3OzMl0OmAVIa2NDJYIDlxMA= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-580-jdWXXdCXP9mMnCa2FnvqhA-1; Thu, 07 Jan 2021 12:36:06 -0500 X-MC-Unique: jdWXXdCXP9mMnCa2FnvqhA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5073C100F349; Thu, 7 Jan 2021 17:36:05 +0000 (UTC) Received: from mail (ovpn-112-222.rdu2.redhat.com [10.10.112.222]) by smtp.corp.redhat.com (Postfix) with ESMTPS id DDEC860C0F; Thu, 7 Jan 2021 17:36:04 +0000 (UTC) Date: Thu, 7 Jan 2021 12:36:04 -0500 From: Andrea Arcangeli To: Vlastimil Babka Cc: Hugh Dickins , Andrew Morton , Alex Shi , Minchan Kim , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/mmap: replace if (cond) BUG() with BUG_ON() Message-ID: References: <1607743586-80303-1-git-send-email-alex.shi@linux.alibaba.com> <1607743586-80303-2-git-send-email-alex.shi@linux.alibaba.com> <20210106114620.5c221690f3a9cad7afcc3077@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/2.0.4 (2020-12-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 07, 2021 at 06:28:29PM +0100, Vlastimil Babka wrote: > On 1/6/21 9:18 PM, Hugh Dickins wrote: > > On Wed, 6 Jan 2021, Andrea Arcangeli wrote: > >> > >> I'd be surprised if the kernel can boot with BUG_ON() defined as "do > >> {}while(0)" so I guess it doesn't make any difference. > > > > I had been afraid of that too, when CONFIG_BUG is not set: > > but I think it's actually "if (cond) do {} while (0)". > > It's a maze of configs and arch-specific vs generic headers, but I do see this > in include/asm-generic/bug.h: > > #else /* !CONFIG_BUG */ > #ifndef HAVE_ARCH_BUG > #define BUG() do {} while (1) > #endif > > So seems to me there *are* configurations possible where side-effects are indeed > thrown away, right? But this not BUG_ON, and that is an infinite loop while(1), not an optimization away as in while (0) that I was suggesting to just throw away cond and make it a noop. BUG() is actually the thing to use to move functional stuff out of BUG_ON so it's not going to be causing issues if it just loops. This overall feels mostly an aesthetically issue. Thanks, Andrea