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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 1FAB6C4332F for ; Thu, 17 Mar 2022 17:47:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Content-Type: Content-Transfer-Encoding:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:From:References:Cc:To:Subject: MIME-Version:Date:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=IQtr6fSx9QMQ6VRqI4ud9Kq0G7rVlmo3NVIpIZu8Tas=; b=F7QhmeJRS+JD95 eBiqkqSvi3NgbC2YJ4ippu1PpzOSIWNGqgUgCjYaeSAi7dlg5hFv2fY7zb1EjD+S4fXOVJ8oPRTyg cLgkSlUx7PseC9NMSuZ5XTqOvl7hWLIcShv2oFjg+HsDESImobp19V9JIaKVYx+I0rcfjXQKEj6HW c0mCqWk6Gi++NPp8LfqSVs5xs7zGM+4uDtNhSjFClUvq5VgmVhrCbDpd0kSY9Uwin+Yr/Rkvrs2vT yivUvHTsDKMayyNyd0Rbp1u0SdxSQpv0oL+Ymj54DI/LQW9JrfQepxb8+fJ+eeOd+9aft/m/AnphO TR8qneOq7ylWZKzahRjw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1nUuDB-00GwTP-3D; Thu, 17 Mar 2022 17:46:57 +0000 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1nUuD7-00GwS4-Up for linux-riscv@lists.infradead.org; Thu, 17 Mar 2022 17:46:56 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1647539213; 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=VoAhd3m5tt2tOFusOR8P0Gc6+W7X3u7kmOYWooMmBwq/5wkqCri72Na+8/QRAifaQmzFlx TFDspP5mhCPcau7RXWkT3SKUvJNSW+MUg2U28gibt9HHD7eVdEsdDHKaO0GYflW6jbObb0 ZYamGQj2Qep3BDVLa+VPSpsvFFWSpsQ= 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> X-Scanned-By: MIMEDefang 2.85 on 10.11.54.10 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220317_104654_078571_F4690453 X-CRM114-Status: GOOD ( 25.58 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.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 _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv