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 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id BBFD4CAC592 for ; Mon, 15 Sep 2025 19:40:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=TiIl36uFY8ZkqiyyqljWp+GeIOt2p0pIoWmVGkzz89Q=; b=NN73Kb2Am1343l0YnRBMxEnhl/ 5nMCkhKJcvXYRBjV1iJZIQ5dRH3SHRm4HKFM69GHSUL+1cDBwvn2lXR7Z9a3MsCXkkISQ5lryEnIS 6rWUjDTeqUWcvDM6lX0l3hKN1yXwCgBMhxD3AvZs50NR4LThjHJvwiE2rVtrEtfUnK4UY7odejHEh W0JIU6y0CWJVGhM7Nrv0MeIQYzTaspYa1uCfPH11/i3h+g0jQfKwUxAKReexg/jN9lSY3HFBL32mZ JKnz1Tl9HgMrOx1tzuWhnWkoIB18E5dP2BCxoFC0bd3PGv+j3+q6pKxkT3IV+jFw8fkPQZbfO7F/e JDOg+x9A==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1uyF45-00000005Woe-27EB; Mon, 15 Sep 2025 19:40:41 +0000 Received: from sea.source.kernel.org ([172.234.252.31]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1uyF43-00000005WoA-1LpX for linux-arm-kernel@lists.infradead.org; Mon, 15 Sep 2025 19:40:40 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id CFF8C43284; Mon, 15 Sep 2025 19:40:38 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2F133C4CEF1; Mon, 15 Sep 2025 19:40:36 +0000 (UTC) Date: Mon, 15 Sep 2025 20:40:33 +0100 From: Catalin Marinas To: Yeoreum Yun Cc: will@kernel.org, broonie@kernel.org, maz@kernel.org, oliver.upton@linux.dev, joey.gouly@arm.com, james.morse@arm.com, ardb@kernel.org, scott@os.amperecomputing.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, mark.rutland@arm.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH RESEND v7 4/6] arm64: futex: refactor futex atomic operation Message-ID: References: <20250816151929.197589-1-yeoreum.yun@arm.com> <20250816151929.197589-5-yeoreum.yun@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250915_124039_391263_AE025DF2 X-CRM114-Status: GOOD ( 23.82 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, Sep 15, 2025 at 11:32:39AM +0100, Yeoreum Yun wrote: > > On Sat, Aug 16, 2025 at 04:19:27PM +0100, Yeoreum Yun wrote: > > > diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h > > > index bc06691d2062..ab7003cb4724 100644 > > > --- a/arch/arm64/include/asm/futex.h > > > +++ b/arch/arm64/include/asm/futex.h > > > @@ -7,17 +7,21 @@ > > > > > > #include > > > #include > > > +#include > > > > > > #include > > > > > > -#define FUTEX_MAX_LOOPS 128 /* What's the largest number you can think of? */ > > > +#define LLSC_MAX_LOOPS 128 /* What's the largest number you can think of? */ > > > > I just noticed - you might as well leave the name as is here, especially > > if in patch 6 you align down address and use CAS on a 64-bit value as > > per Will's comment (and it's no longer LLSC). I think renaming this is > > unnecessary. > > Okay. I'll restore to use origin name. > But I think LSUI wouldn't be used with CAS according to patch 6's > comments from you and additionally i think > chaning the CAS would make a failure because of > change of unrelated field. i.e) > > struct user_structure{ > uint32 futex; > uint32 some_value; > }; > > In this case, the change of some_value from user side could make a > failure of futex atomic operation. Yes but the loop would read 'some_value' again, fold in 'futex' and retry. It would eventually succeed or fail after 128 iterations if the user keeps changing that location. Note that's also the case with LL/SC, the exclusive monitor would be cleared by some store in the same cache line (well, depending on the hardware implementation) and the STXR fail. >From this perspective, CAS has better chance of succeeding. > So I think it would be better to keep the current LLSC implementation > in LSUI. I think the code would look simpler with LL/SC but you can give it a try and post the code sample here (not in a new series). BTW, is there a test suite for all the futex operations? The cover letter did not mention any. -- Catalin