From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrzej Hajda Subject: Re: [PATCH 00/19] Introduce __xchg, non-atomic xchg Date: Thu, 29 Dec 2022 10:54:50 +0100 Message-ID: <6e727952-3ad0-fcc3-82f1-c465dcffd56f@intel.com> References: <20221222114635.1251934-1-andrzej.hajda@intel.com> <20221222092147.d2bb177c67870884f2e59a9b@linux-foundation.org> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1672307698; x=1703843698; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=Ekul8eGeWcmCvyM+3dUi16jfyX+OFQQRDZLG3bQVomQ=; b=d+7bN+tjqh3eof9bdGje2kpI4zamfY6oXHKsH+IltfywydlGu2NF0yZG 4vIYv/W8lJbY30ukqxMBcjHIRPMCg9jmTJADrPDZdo1FJxI3F6MVBRfwY BIgsMVcxs9JW7VBN0D8cQHYX+iVJvNWG3lWD/FemsFFxxPHFlM+r1fbYY qVh8BxuHJ89OYNeXwCreHJSPhBe+76NSoPDLdOu7BGq1oDzOlJyyMm8DT Ef4uT6gE+ggVAwsKhrxH9vy9WHVpZ+HRJiPSnjy/8deUA37o+bNIcgSHi 0elXSLbGnfbt6xofms5/X0zBh+1Hn27+PPdr9nHZYPjJGtGv4Kw4SFjyF g==; Content-Language: en-US In-Reply-To: <20221222092147.d2bb177c67870884f2e59a9b@linux-foundation.org> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" Content-Type: text/plain; charset="utf-8"; format="flowed" To: Andrew Morton Cc: Mark Rutland , linux-m68k@vger.kernel.org, linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org, Peter Zijlstra , dri-devel@lists.freedesktop.org, linux-mips@vger.kernel.org, sparclinux@vger.kernel.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-snps-arc@lists.infradead.org, Boqun Feng , linux-xtensa@linux-xtensa.org, Arnd Bergmann , intel-gfx@lists.freedesktop.org, openrisc@lists.librecores.org, loongarch@lists.linux.dev, Rodrigo Vivi , Andy Shevchenko , linux-arm-kernel@lists.infradead.org, linux-parisc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Forgive me late response - Holidays, On 22.12.2022 18:21, Andrew Morton wrote: > On Thu, 22 Dec 2022 12:46:16 +0100 Andrzej Hajda wrote: > >> Hi all, >> >> I hope there will be place for such tiny helper in kernel. >> Quick cocci analyze shows there is probably few thousands places >> where it could be useful. > So to clarify, the intent here is a simple readability cleanup for > existing open-coded exchange operations. And replace private helpers with common one, see the last patch - the ultimate goal would be to replace all occurrences of fetch_and_zero with __xchg. > The intent is *not* to > identify existing xchg() sites which are unnecessarily atomic and to > optimize them by using the non-atomic version. > > Have you considered the latter? If you mean some way of (semi-)automatic detection of such cases, then no. Anyway this could be quite interesting challenge. > >> I am not sure who is good person to review/ack such patches, > I can take 'em. > >> so I've used my intuition to construct to/cc lists, sorry for mistakes. >> This is the 2nd approach of the same idea, with comments addressed[0]. >> >> The helper is tiny and there are advices we can leave without it, so >> I want to present few arguments why it would be good to have it: >> >> 1. Code readability/simplification/number of lines: >> >> Real example from drivers/net/ethernet/mellanox/mlx5/core/esw/qos.c: >> - previous_min_rate = evport->qos.min_rate; >> - evport->qos.min_rate = min_rate; >> + previous_min_rate = __xchg(evport->qos.min_rate, min_rate); >> >> For sure the code is more compact, and IMHO more readable. >> >> 2. Presence of similar helpers in other somehow related languages/libs: >> >> a) Rust[1]: 'replace' from std::mem module, there is also 'take' >> helper (__xchg(&x, 0)), which is the same as private helper in >> i915 - fetch_and_zero, see latest patch. >> b) C++ [2]: 'exchange' from utility header. >> >> If the idea is OK there are still 2 qestions to answer: >> >> 1. Name of the helper, __xchg follows kernel conventions, >> but for me Rust names are also OK. > I like replace(), or, shockingly, exchange(). > > But... Can we simply make swap() return the previous value? > > previous_min_rate = swap(&evport->qos.min_rate, min_rate); As Alexander already pointed out, swap requires 'references' to two variables, in contrast to xchg which requires reference to variable and value. So we cannot use swap for cases:     old_value = __xchg(&x, new_value); Regards Andrzej