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 8D60EC433EF for ; Thu, 17 Mar 2022 17:47:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237058AbiCQRsV (ORCPT ); Thu, 17 Mar 2022 13:48:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58452 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237081AbiCQRsS (ORCPT ); Thu, 17 Mar 2022 13:48:18 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id E2F591F6F15 for ; Thu, 17 Mar 2022 10:46:55 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1647539215; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=llqjR9J1FmybTT0pHxsozwhPdyLYqoq+OgcFl5fMvI4=; b=W6Z+VFxTqMJjG986WwXZeCTShLGTa4JjoyItwsT7jfRu838qXWwT/PxJwwuh2zfzIB00Pd OSDrAbxF4Z+7XQ95MYeC6eIDx669LourM67FWbkLhVvFb0bwtS7LNrH4UYTT8GeBURHGut DZn9coZMXrSZELjThptPPih0hiJbXpk= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-201-D_1JD0DkM1yNyRBj3b6jGg-1; Thu, 17 Mar 2022 13:46:49 -0400 X-MC-Unique: D_1JD0DkM1yNyRBj3b6jGg-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 8A59218A6584; Thu, 17 Mar 2022 17:46:48 +0000 (UTC) Received: from [10.22.8.95] (unknown [10.22.8.95]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4EC33401E87; Thu, 17 Mar 2022 17:46:47 +0000 (UTC) Message-ID: <67ba2190-dd72-4ad0-32c2-de43418b73a2@redhat.com> Date: Thu, 17 Mar 2022 13:46:46 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [PATCH 1/5] asm-generic: qspinlock: Indicate the use of mixed-size atomics Content-Language: en-US To: Palmer Dabbelt , linux-riscv@lists.infradead.org, peterz@infradead.org Cc: jonas@southpole.se, stefan.kristiansson@saunalahti.fi, shorne@gmail.com, mingo@redhat.com, Will Deacon , boqun.feng@gmail.com, Paul Walmsley , Palmer Dabbelt , aou@eecs.berkeley.edu, Arnd Bergmann , jszhang@kernel.org, wangkefeng.wang@huawei.com, openrisc@lists.librecores.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org References: <20220316232600.20419-1-palmer@rivosinc.com> <20220316232600.20419-2-palmer@rivosinc.com> From: Waiman Long In-Reply-To: <20220316232600.20419-2-palmer@rivosinc.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.85 on 10.11.54.10 Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On 3/16/22 19:25, Palmer Dabbelt wrote: > From: Peter Zijlstra > > The qspinlock implementation depends on having well behaved mixed-size > atomics. This is true on the more widely-used platforms, but these > requirements are somewhat subtle and may not be satisfied by all the > platforms that qspinlock is used on. > > Document these requirements, so ports that use qspinlock can more easily > determine if they meet these requirements. > > Signed-off-by: Palmer Dabbelt > > --- > > I have specifically not included Peter's SOB on this, as he sent his > original patch > > without one. > --- > include/asm-generic/qspinlock.h | 30 ++++++++++++++++++++++++++++++ > 1 file changed, 30 insertions(+) > > diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h > index d74b13825501..a7a1296b0b4d 100644 > --- a/include/asm-generic/qspinlock.h > +++ b/include/asm-generic/qspinlock.h > @@ -2,6 +2,36 @@ > /* > * Queued spinlock > * > + * A 'generic' spinlock implementation that is based on MCS locks. An > + * architecture that's looking for a 'generic' spinlock, please first consider > + * ticket-lock.h and only come looking here when you've considered all the > + * constraints below and can show your hardware does actually perform better > + * with qspinlock. > + * > + * > + * It relies on atomic_*_release()/atomic_*_acquire() to be RCsc (or no weaker > + * than RCtso if you're power), where regular code only expects atomic_t to be > + * RCpc. > + * > + * It relies on a far greater (compared to ticket-lock.h) set of atomic > + * operations to behave well together, please audit them carefully to ensure > + * they all have forward progress. Many atomic operations may default to > + * cmpxchg() loops which will not have good forward progress properties on > + * LL/SC architectures. > + * > + * One notable example is atomic_fetch_or_acquire(), which x86 cannot (cheaply) > + * do. Carefully read the patches that introduced queued_fetch_set_pending_acquire(). > + * > + * It also heavily relies on mixed size atomic operations, in specific it > + * requires architectures to have xchg16; something which many LL/SC > + * architectures need to implement as a 32bit and+or in order to satisfy the > + * forward progress guarantees mentioned above. > + * > + * Further reading on mixed size atomics that might be relevant: > + * > + * http://www.cl.cam.ac.uk/~pes20/popl17/mixed-size.pdf > + * > + * > * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P. > * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP > * Acked-by: Waiman Long Note that it references ticket-lock.h. Perhaps we should reverse the order of patches 1 & 2. Cheers, Longman