From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mgamail.intel.com (mgamail.intel.com [198.175.65.12]) (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 B237B15C14F; Wed, 10 Jun 2026 14:43:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.175.65.12 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781102612; cv=none; b=o4fphUl6PeXOgsJWQtxVS7qXgoKbZg6tWQiAKCEGysHjQ8zJOpFXx9FJ6AH5sjqZByM4X4xdfz5a5S3Is4CAzAeBZ2f+4m9VAKsoVwGTT74URXz6+I6bUedwZ8cj5XZFVs5ELQvek7RzoEulE3fbwru3cMgPcdXvbxKs4OKnRkE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781102612; c=relaxed/simple; bh=DPrzoI0Cic4lctVW9xW26ap2kbgPfsaWv6mpsae+kMQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=EarYmbdRF5vKGtm3ZXBS4R/7g27LPIzt1HzMWv0Tagr0rNl7uOPkOrDtH2goVYoTXs5y3BEZsp7yq9CCprQHoZ/raYM+1jG4SJVJwrNF7RgaAhvpSd+gK2NYCIwPimaL2b1zu6U6eqpr4tAItkZtlA4NQjvv/wq2ZN3VUq3zVHc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.intel.com; spf=pass smtp.mailfrom=linux.intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=na+2ujzO; arc=none smtp.client-ip=198.175.65.12 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="na+2ujzO" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1781102611; x=1812638611; h=date:from:to:cc:subject:message-id:references: mime-version:content-transfer-encoding:in-reply-to; bh=DPrzoI0Cic4lctVW9xW26ap2kbgPfsaWv6mpsae+kMQ=; b=na+2ujzOI/QIK0hxH5SmVq1OXSr79K7m/H8isj9MQ5AmNt4nlE1DxXup fPZ7F9YdTnOBBxSPzOnBbKXlJF5Br8qmL1gV0OuqpbLLnwen2c/pDHV88 WWNolPd0zbKNXp1YioXUxrea6e8337OOgT6CBDS6upRxVlzK1RuJRLKSW 2F48BTWvOORjkFqlmDzLwfH+rOlRC8xbOehdLaQCWv90whbVrLUvwUd8d Iwp2MulVgI7UWcmRn2Pml5FhEeCjynitbdmEu+2+04tysfUi8Mu+nEnIb r0we5NcGUpXxx59lbyuJrfR5fCgkVXe5/tP8rYdYBhTmcQ5k4VWcL76UW w==; X-CSE-ConnectionGUID: hmGqtfgORD6gGQuC8j5eCw== X-CSE-MsgGUID: T8q1HyzvTraFe67iTMRHhg== X-IronPort-AV: E=McAfee;i="6800,10657,11813"; a="93384023" X-IronPort-AV: E=Sophos;i="6.24,197,1774335600"; d="scan'208";a="93384023" Received: from orviesa005.jf.intel.com ([10.64.159.145]) by orvoesa104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 10 Jun 2026 07:43:30 -0700 X-CSE-ConnectionGUID: 9i25flRDSViVTbAIRpyEPA== X-CSE-MsgGUID: qyMNFvgnTEePAxuMiNBn1g== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.24,197,1774335600"; d="scan'208";a="251104880" Received: from hrotuna-mobl2.ger.corp.intel.com (HELO localhost) ([10.245.244.38]) by orviesa005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 10 Jun 2026 07:43:17 -0700 Date: Wed, 10 Jun 2026 17:43:14 +0300 From: Andy Shevchenko To: Kaitao Cheng Cc: Christian =?iso-8859-1?Q?K=F6nig?= , Thierry Reding , Jonathan Hunter , Sowjanya Komatineni , Davidlohr Bueso , "Paul E . McKenney" , Josh Triplett , Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng , Liam Girdwood , Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , Tvrtko Ursulin , Huang Rui , Eddie James , Mark Brown , Maxime Coquelin , Alexandre Torgue , Laxman Dewangan , Neil Armstrong , Robert Foss , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Laurent Pinchart , Jonas Karlman , Jernej Skrabec , Matthew Auld , Matthew Brost , Waiman Long , drbd-dev@lists.linbit.com, linux-block@vger.kernel.org, linux1394-devel@lists.sourceforge.net, dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, linux-spi@vger.kernel.org, linux-stm32@st-md-mailman.stormreply.com, linux-arm-kernel@lists.infradead.org, linux-tegra@vger.kernel.org, linux-sound@vger.kernel.org, linux-kernel@vger.kernel.org, Andrew Morton , Randy Dunlap , Christian Brauner , David Howells , Luca Ceresoli , Kaito Cheng , Muchun Song , Philipp Reisner , Lars Ellenberg , Christoph =?iso-8859-1?Q?B=F6hmwalder?= , Jens Axboe , Takashi Sakamoto , Andrzej Hajda , Jaroslav Kysela , Takashi Iwai Subject: Re: [PATCH v2 00/14] list: Prepare entry iterators to cache cursor state Message-ID: References: <20260609061347.93688-1-kaitao.cheng@linux.dev> <5152089a-2808-4fe9-b633-b03018105dd2@linux.dev> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <5152089a-2808-4fe9-b633-b03018105dd2@linux.dev> Organization: Intel Finland Oy - BIC 0357606-4 - c/o Alberga Business Park, 6 krs, Bertel Jungin Aukio 5, 02600 Espoo On Wed, Jun 10, 2026 at 02:14:06PM +0800, Kaitao Cheng wrote: > 在 2026/6/9 18:33, Christian König 写道: > > On 6/9/26 08:13, Kaitao Cheng wrote: > >> > >> This series prepares for, and then updates, the list_for_each_entry() > >> family so the common entry iterators cache their next or previous cursor > >> before the loop body runs. > > > > Why in the world would we want to do that? > > > > The safe and non-safe variants have very distinct use cases and that is completely intentional. > > > > What we could improve maybe is the documentation, from my experience an astonishing large amount of people have misconceptions about the safe variants. > > > >> The first 13 patches open-code loops that intentionally depend on the > >> old "derive the next entry from the current cursor at the end of the > >> iteration" behaviour. These loops append work to the list being walked, > >> restart traversal after dropping a lock, skip an entry consumed by the > >> current iteration, or otherwise adjust the cursor in the loop body. > > > > Well I have to clearly reject the changes for subsystems/components I'm maintaining, that just looks horrible to me and I clearly don't see a good reason for that. > > Hi Christian and Andy Shevchenko, > > Thanks for taking a look. I would like to clarify the point you raised. > > The reason I started looking at this is the original motivation behind > the _safe() variants. They exist because some users need to remove, move > or otherwise consume the current entry while walking the list. In that > case the next cursor has to be preserved before the loop body can modify > the current entry. > > The unfortunate part is that this could not be expressed with the > existing list_for_each_entry() interface without changing its calling > convention. The _safe() variants had to grow an extra argument for the > temporary cursor, and that is why we ended up with a separate family of > macros. > > But conceptually, the distinction does not have to be exposed as two > different iterator families forever. The difference is an implementation > detail: whether the iterator keeps the next/previous cursor before the > body runs. This series makes the common list_for_each_entry() iterators > do that internally, so the safe and non-safe forms can effectively be > folded together, or at least the need for a separate public _safe() > interface becomes much weaker. > > There is also a usability issue with the current _safe() interface. The > caller is forced to define a temporary cursor outside the macro and pass > it in, even though almost all users never use that cursor directly. It is > just boilerplate required by the macro implementation. I find that > redundant and awkward: the temporary cursor is an internal detail of the > iteration, but every caller has to spell it out. Ah, I think the distinct macro families is that what we want. But the hiding of the parameter can be done inside list_for_each_*_safe(). You can do a treewide change with coccinelle. Sorry if I didn't get the whole idea from your previous contributions. Note, even cases that would need a temporary cursor may be switched to new list_for_each_*_safe(), see how PCI macros for iterating over resources are implemented (include/linux/pci.h). > With the updated list_for_each_entry() implementation, that extra cursor > can be kept inside the iterator itself. Callers that only want to walk > the list, including callers that delete or consume the current entry, no > longer need to carry an otherwise-unused temporary variable just to make > the macro work. > > >> The final patch changes include/linux/list.h to keep a private cursor in > >> the common entry iterators while preserving the public macro interface. > >> The safe variants remain available when callers need the temporary > >> cursor explicitly or have stronger mutation requirements. -- With Best Regards, Andy Shevchenko