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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B5B27C53210 for ; Thu, 5 Jan 2023 14:26:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233661AbjAEO0p (ORCPT ); Thu, 5 Jan 2023 09:26:45 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36920 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233520AbjAEO0I (ORCPT ); Thu, 5 Jan 2023 09:26:08 -0500 Received: from mga18.intel.com (mga18.intel.com [134.134.136.126]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EFF894A964 for ; Thu, 5 Jan 2023 06:26:06 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1672928767; x=1704464767; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=z67iM0jfw8rBDBXuutR2adWOVf2bd9BhkqQyZcE/mpY=; b=bJmGz2yuwoesCJg/qlfR+WaB0Hp24qAjwrrmVH7ARqefSTdmWmO25P98 S9YogtixSUWanBlaOqyElRirgv9IH92DUmkMDoVq7YUaZEGFnBPRffTcG x7nPAi5FznUx/emmoDV2trFYlwnFOOSNP+l/gIQsT7M23ZiPeW8BxS0GO Wg1LQm/RVK5pGMoJFHzuy9lemXk/F3ho3LDJ2Ge5LHx2ZPer0IBpXtftE MzQYpnM+B3BU4lmflHwgFYbvpLW+AzZk1/JSOQlqyQjM/zDXtY5sWq50l WbL8C5cjQ+mZdP+9JI00j9G14SfzIwPPmNqVScjls65+ahpMAu5zR/wZN A==; X-IronPort-AV: E=McAfee;i="6500,9779,10580"; a="305721809" X-IronPort-AV: E=Sophos;i="5.96,303,1665471600"; d="scan'208";a="305721809" Received: from orsmga005.jf.intel.com ([10.7.209.41]) by orsmga106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 05 Jan 2023 06:13:50 -0800 X-IronPort-AV: E=McAfee;i="6500,9779,10580"; a="829564429" X-IronPort-AV: E=Sophos;i="5.96,303,1665471600"; d="scan'208";a="829564429" Received: from ciarapow-mobl1.ger.corp.intel.com (HELO [10.213.208.73]) ([10.213.208.73]) by orsmga005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 05 Jan 2023 06:13:47 -0800 Message-ID: Date: Thu, 5 Jan 2023 14:13:45 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.1 Subject: Re: [Intel-gfx] [PATCH 1/5] linux/minmax.h: add non-atomic version of xchg Content-Language: en-US To: David Laight , 'Jani Nikula' , Daniel Vetter Cc: Arnd Bergmann , "intel-gfx@lists.freedesktop.org" , "linux-kernel@vger.kernel.org" , "dri-devel@lists.freedesktop.org" , 'Andrzej Hajda' , Rodrigo Vivi , Andrew Morton , Andy Shevchenko References: <20221209154843.4162814-1-andrzej.hajda@intel.com> <875ydlw1p4.fsf@intel.com> <733cd0037bd14a269b54d701e1b80323@AcuMS.aculab.com> From: Tvrtko Ursulin Organization: Intel Corporation UK Plc In-Reply-To: <733cd0037bd14a269b54d701e1b80323@AcuMS.aculab.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/01/2023 13:34, David Laight wrote: > From: Jani Nikula >> Sent: 05 January 2023 13:28 >> >> On Thu, 05 Jan 2023, Daniel Vetter wrote: >>> On Mon, Dec 12, 2022 at 09:38:12AM +0000, David Laight wrote: >>>> From: Andrzej Hajda >>>>> Sent: 09 December 2022 15:49 >>>>> >>>>> The pattern of setting variable with new value and returning old >>>>> one is very common in kernel. Usually atomicity of the operation >>>>> is not required, so xchg seems to be suboptimal and confusing in >>>>> such cases. Since name xchg is already in use and __xchg is used >>>>> in architecture code, proposition is to name the macro exchange. >>>> >>>> Dunno, if it is non-atomic then two separate assignment statements >>>> is decidedly more obvious and needs less brain cells to process. >>>> Otherwise someone will assume 'something clever' is going on >>>> and the operation is atomic. >>> >>> Yes, this also my take. The i915 code that uses this to excess is decidely >>> unreadable imo, and the macro should simply be replaced by open-coded >>> versions. >>> >>> Not moved into shared headers where even more people can play funny games >>> with it. >> >> My stand in i915 has been that the local fetch_and_zero() needs to >> go. Either replaced by a common helper in core kernel headers, or open >> coded, I personally don't care, but the local version can't stay. >> >> My rationale has been that fetch_and_zero() looks atomic and looks like >> it comes from shared headers, but it's neither. It's deceptive. It >> started small and harmless, but things like this just proliferate and >> get copy-pasted all over the place. >> >> So here we are, with Andrzej looking to add the common helper. And the >> same concerns crop up. What should it be called to make it clear that >> it's not atomic? Is that possible? > > old_value = read_write(variable, new_value); > > But two statements are much clearer. In a later thread there was more discussion on this and some new suggestions - exchange(), replace() or even take() sound fine to me. Last one is perhaps most specialized if it implies zeroing, which I at least assume it does. All three are distant enough from atomic connotations of xchg. If that was a concern with __xchg, which I not sure it should be since there is "prior art" in the kernel for atomic vs non-atomic like set_bit and __set_bit. My 2c, regardless of what name, that it is not something which is strictly needed, but a convenient syntactic sugar. (Exploded line counts with sometimes single use local variables are a bit meh.) And I am not really sure that open coding is more readable once the new pattern would be established. In short, if there can be swap there can be $insert_name too I guess. Bonus points if needlessly atomic sites can be converted but identifying them is probably an exercise for a later phase. Regards, Tvrtko P.S. FWIW my preference are either replace() or __xchg().