From owner-svn-src-all@FreeBSD.ORG Mon Feb 9 09:03:21 2015 Return-Path: Delivered-To: svn-src-all@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id A63AF97; Mon, 9 Feb 2015 09:03:21 +0000 (UTC) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 1834E61; Mon, 9 Feb 2015 09:03:20 +0000 (UTC) Received: from tom.home (kostik@localhost [127.0.0.1]) by kib.kiev.ua (8.14.9/8.14.9) with ESMTP id t1993F6W010954 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Mon, 9 Feb 2015 11:03:15 +0200 (EET) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.9.2 kib.kiev.ua t1993F6W010954 Received: (from kostik@localhost) by tom.home (8.14.9/8.14.9/Submit) id t1993F1j010953; Mon, 9 Feb 2015 11:03:15 +0200 (EET) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Mon, 9 Feb 2015 11:03:15 +0200 From: Konstantin Belousov To: Bruce Evans Subject: Re: svn commit: r278431 - head/sys/contrib/vchiq/interface/vchiq_arm Message-ID: <20150209090315.GD42409@kib.kiev.ua> References: <201502090231.t192VS6C060751@svn.freebsd.org> <20150209170045.S1037@besplex.bde.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150209170045.S1037@besplex.bde.org> User-Agent: Mutt/1.5.23 (2014-03-12) X-Spam-Status: No, score=-2.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FREEMAIL_FROM,NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.0 X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on tom.home Cc: svn-src-head@freebsd.org, Oleksandr Tymoshenko , svn-src-all@freebsd.org, src-committers@freebsd.org X-BeenThere: svn-src-all@freebsd.org X-Mailman-Version: 2.1.18-1 Precedence: list List-Id: "SVN commit messages for the entire src tree \(except for " user" and " projects" \)" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 09 Feb 2015 09:03:21 -0000 On Mon, Feb 09, 2015 at 05:00:49PM +1100, Bruce Evans wrote: > On Mon, 9 Feb 2015, Oleksandr Tymoshenko wrote: > > > Log: > > Do not mark shared structures as __packed, it leads to race condition > > > > If structure packed as __packed clang (and probably gcc) generates > > code that loads word fields (e.g. tx_pos) byte-by-byte and if it's > > modified by VideoCore in the same time as ARM loads the value result > > is going to be mixed combination of bytes from previous value and > > new one. > > Most uses of __packed are bugs. It gives pessimizations as well as > non-atomic accesses for sub-object accesses. > > I think the full bugs only occur when arch has strict alignment > requirements and the alignment of the __packed objects is not known. > This means that only lesser bugs occur on x86 (unless you enable > alignment checking, but this arguably breaks the ABI). The compiler > just generates possibly-misaligned full-width accesses if the arch > doesn't have strict alignment requirements. Often the acceses turn > out to be aligned at runtime. Otherwise, the hardware does them > atomically, with a smaller efficiency penalty than split accesses. On x86 unaligned access is non-atomic. This was very visible on Core2 CPUs where DPCPU code mishandled the alignment, resulting in the mutexes from the per-cpu areas breaking badly. Modern CPUs should not lock several cache lines simultaneously either.