From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexander Lobakin Subject: Re: [PATCH 00/19] Introduce __xchg, non-atomic xchg Date: Fri, 23 Dec 2022 15:23:00 +0100 Message-ID: <20221223142300.1820652-1-alexandr.lobakin@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=1671805399; x=1703341399; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version:content-transfer-encoding; bh=ELTp1mLG7Y9aCL8aO+3yggqOoksNOYkU+LC/pmwgzyA=; b=h6WHojs6ZpmqWpG2ottoZdl2pYgLLN2M3eW5A5tU9xF/OQlsT/3lk21H c+27dZk3j5L1dN6yN+xTy/Nw2p/XcwLUfb5INN1T1g/0ViZeKJo5dasVa 2aM6Qe9tY1i/8kBLtimzGb9etHds0Vj1ICf1c6EWPlU8veg+mh8GlRw1f H1RVwid1/AgXqzvKKGomudHa9NdK5cWPCTVhnP/tSl3PViN1cgTQlPXxG AQqDeqzUnhnC4/YrPK4vlFEzcvUOFF7jMGVaZIH/9U4/Af3L+SOw42Fzh ybnz3reqKRmJRugCywjpmYvBfkXpozGjqKHcppjmrXomjjgBb2U1Jx0jJ g==; In-Reply-To: <20221222092147.d2bb177c67870884f2e59a9b@linux-foundation.org> List-ID: Content-Type: text/plain; charset="us-ascii" To: Andrew Morton Cc: Alexander Lobakin , Andrzej Hajda , linux-alpha@vger.kernel.org, linux-kernel@vger.kernel.org, linux-snps-arc@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org, loongarch@lists.linux.dev, linux-m68k@vger.kernel.org, linux-mips@vger.kernel.org, openrisc@lists.librecores.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org, intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, Arnd Bergmann , Rodrigo Vivi , Andy Shevchenko From: Andrew Morton Date: Thu, 22 Dec 2022 09:21:47 -0800 > 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. 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? > > > 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); Unforunately, swap()'s arguments get passed by names, not as pointers, so you can't do swap(some_ptr, NULL); -- pretty common pattern for xchg. Thanks, Olek