From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:59076 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727264AbfJVBpG (ORCPT ); Mon, 21 Oct 2019 21:45:06 -0400 Date: Mon, 21 Oct 2019 18:45:03 -0700 From: Eric Biggers Subject: Re: arch/riscv doesn't support xchg() on bool Message-ID: <20191022014503.GA938@sol.localdomain> References: <20191021204026.GE122863@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-fscrypt-owner@vger.kernel.org To: Paul Walmsley Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, linux-fscrypt@vger.kernel.org List-ID: Hi Paul, On Mon, Oct 21, 2019 at 06:23:11PM -0700, Paul Walmsley wrote: > Hi Eric, > > On Mon, 21 Oct 2019, Eric Biggers wrote: > > > The kbuild test robot reported a build error on RISC-V in this patch: > > > > https://patchwork.kernel.org/patch/11182389/ > > > > ... because of the line: > > > > if (!xchg(&mode->logged_impl_name, true)) { > > > > where logged_impl_name is a 'bool'. The problem is that unlike most (or > > all?) other kernel architectures, arch/riscv/ doesn't support xchg() on > > bytes. > > When I looked at this in August, it looked like several Linux other > architectures - SPARC, Microblaze, C-SKY, and Hexagon - also didn't > support xchg() on anything other than 32-bit types: > > https://lore.kernel.org/lkml/alpine.DEB.2.21.9999.1908161931110.32497@viisi.sifive.com/ > > Examples: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/sparc/include/asm/cmpxchg_32.h#n18 > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/sparc/include/asm/cmpxchg_32.h#n41 > > > Is there any chance this could be implemented, to avoid this > > architecture-specific quirk? > > It is certainly possible. I wonder whether it is wise. Several of the > other architectures implement a software workaround for this operation, > and I guess you're advocating that we do the same. We could copy one > these implementations. However, the workarounds balloon into quite a lot > of code. Here is an example from MIPS: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/mips/kernel/cmpxchg.c#n10 > > I could be wrong, but I think this expansion would be pretty surprising > for most users of xchg(). I suspect most xchg() users are looking for > something performant, and would be better served by simply using a > variable with a 32-bit type. > > In the case of your patch, it appears that struct > fscrypt_mode.logged_impl_name is only used in the patched function. It > looks like it could be promoted into a u32 without much difficulty. > Would you be willing to consider that approach of solving the problem? > Then the code would be able to take advantage of the fast hardware > implementation that's available on many architectures (including RISC-V). Yes, I already sent a new version of the patch, which changes the variable to an int: https://patchwork.kernel.org/patch/11203003/. I was wondering more about how to stop other people from running into this. > > > Note, there's at least one other place in the kernel that also uses > > xchg() on a bool. > > Given the nasty compatibility code, I wonder if we'd be better served by > removing most of this compatibility code across the kernel, and just > requiring callers to use a 32-bit type? For most callers that I've seen, > this doesn't seem to be much of an issue; and it would avoid the nasty > code involved in software emulations of xchg(). > It's possible that's the better approach; someone would need to go through all the xchg() users and check whether any truly need the 8 or 16-bit support. My main concern was just the annoyance of code that only fails to compile on certain architectures. It should either be one way or the other everywhere. - Eric