From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.mainlining.org (mail.mainlining.org [5.75.144.95]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 61E684BEE2A; Sat, 12 Sep 2026 10:31:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=5.75.144.95 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789209102; cv=none; b=PJHt56CZaqiogyRNuY1SH/emhgrXN0aVeba5rvJ+cLCFSMbW/ZWV9THUVYl5zVTn0jhQARMIDz/ZoKiWNtRGRQ8eV6sWCmzrOFVnwR4aIAA9Agrq9saQjOeIoTcbJB0aG0fBXDSuuLYSh11ZwJFRMGLWF8aHm3GZMu7Ozy+Gdn4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789209102; c=relaxed/simple; bh=e5kZ0aoENwpriGEe0youjd9NbKIMnneVpfZH4WKrAZY=; h=Date:From:To:CC:Subject:In-Reply-To:References:Message-ID: MIME-Version:Content-Type; b=iOnmIecFYTGen9JOtpGZBr8kPKDUVub16E5PjX8hPAZPhHBFDjoPf58F/J4Day5GwUk3OyNRaapfO0zA1x96kqVJXodnde4t3bEJ4vhXo64yjGsmK+ODYmkT+2mcmDIQxgfbBxNx/9X/vZaUO1BRnu8oGH5j/P5jCODsSRGvkIg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=mainlining.org; spf=pass smtp.helo=mail.mainlining.org; arc=none smtp.client-ip=5.75.144.95 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=mainlining.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.helo=mail.mainlining.org Date: Sat, 12 Sep 2026 11:29:03 +0100 From: Bradley Morgan To: David Laight , "Paul E. McKenney" CC: frederic@kernel.org, neeraj.upadhyay@kernel.org, boqun@kernel.org, joelagnelf@nvidia.com, rcu@vger.kernel.org, Andrew Morton , Arnd Bergmann , linux-arch@vger.kernel.org, Vineet Gupta , linux-snps-arc@lists.infradead.org, Russell King , linux-arm-kernel@lists.infradead.org, Guo Ren , linux-csky@vger.kernel.org, Yoshinori Sato , Rich Felker , John Paul Adrian Glaubitz , linux-sh@vger.kernel.org, Chris Zankel , Max Filippov , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/6] lib: Add two-byte cmpxchg emulation function In-Reply-To: <20260912103047.1b95bb03@pumpkin> References: <20260911192540.20983-1-brads@mainlining.org> <20260911192540.20983-2-brads@mainlining.org> <20260911231017.550f8cd1@pumpkin> <20260912103047.1b95bb03@pumpkin> Message-ID: <970C9FF6-1D56-48CA-8C2E-1DD332391229@mainlining.org> Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit On 12 September 2026 10:30:47 BST, David Laight wrote: >On Fri, 11 Sep 2026 16:09:16 -0700 >"Paul E. McKenney" wrote: > >> On Fri, Sep 11, 2026 at 11:10:17PM +0100, David Laight wrote: >> > On Fri, 11 Sep 2026 19:25:34 +0000 >> > Bradley Morgan wrote: >> > >> > > cmpxchg_emu_u8() emulates one-byte cmpxchg() in terms of four-byte >> > > cmpxchg() for the architectures lacking native one-byte atomics. >> > > The same architectures also lack native two-byte cmpxchg(), where >> > > such an operation is not supported and either fails to compile via >> > > BUILD_BUG() or fails to link, because the bad pointer sentinels >> > > these architectures declare are never defined. >> > > >> > > Add cmpxchg_emu_u16(), the two-byte sibling. It reads the enclosing >> > > word with READ_ONCE(), splices the two target bytes through a union >> > > and loops on cmpxchg() of the full word until the compare succeeds. >> > > Like cmpxchg_emu_u8() it is fully ordered. >> > > >> > > The Kconfig symbol gating this file is renamed from >> > > ARCH_NEED_CMPXCHG_1_EMU to ARCH_NEED_CMPXCHG_1_2_EMU, as it now >> > > selects both the one-byte and the two-byte emulation. >> > > >> > > Suggested-by: Paul E. McKenney >> > > Signed-off-by: Bradley Morgan >> > > --- >> > > arch/Kconfig | 2 +- >> > > arch/arc/Kconfig | 2 +- >> > > arch/arm/Kconfig | 2 +- >> > > arch/csky/Kconfig | 2 +- >> > > arch/sh/Kconfig | 2 +- >> > > arch/xtensa/Kconfig | 2 +- >> > > include/linux/cmpxchg-emu.h | 1 + >> > > lib/Makefile | 2 +- >> > > lib/cmpxchg-emu.c | 35 >++++++++++++++++++++++++++++++++--- >> > > 9 files changed, 40 insertions(+), 10 deletions(-) >> > > >> > ... >> > > diff --git a/include/linux/cmpxchg-emu.h >b/include/linux/cmpxchg-emu.h >> > > index 998deec67740..fee8171fa05e 100644 >> > > --- a/include/linux/cmpxchg-emu.h >> > > +++ b/include/linux/cmpxchg-emu.h >> > > @@ -11,5 +11,6 @@ >> > > #define __LINUX_CMPXCHG_EMU_H >> > > >> > > uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t >new); >> > > +uintptr_t cmpxchg_emu_u16(volatile u16 *p, uintptr_t old, uintptr_t >new); >> > >> > Why uintptr_t? Shouldn't it just be u16? >> > (Which probably means the code would better if it was just 'unsigned >int') >> >> I suspect that Bradley is just following my cmpxchg_emu_u8() example, >> which also returns uintptr_t. >> >> I remember that *something* broke when I made this be u8, but I cannot >> recall what the problem was. >> >> Bradley, could you please try making it be u16 as David suggests just to >> see what happens? Who knows? Maybe it was a compiler issue that has >> since been fixed. Or maybe the macros and asms using cmpxchg_emu_u8() >> need that uintptr_t for some reason. > >I think the uintptr (unsigned long) cast is needed to stop a compile >error when exchanging pointers. >But that is an issue with the #define not the called functions. > >Possibly changing the #define to have: > unsigned long ul_old = (unsigned long)(old); >Or even, with the type check from: > unsigned long ul_old = (unsigned long)(0 ? *(ptr) : (old)); >(with the same for 'new') >and the removing all the casts where the value are used might be better. > >David > >> >> Thanx, Paul > > Hmm. Okie dokie, noted for V2, thanks for the review, I'm sure arch code is just implementing this function, so I hope nothing went wrong with the arch code. --- Thanks! https://lore.kernel.org/all/EE579805-42F2-4C58-B752-F28779EEB717@grrlz.net/