From owner-svn-src-head@FreeBSD.ORG Sat Jan 24 15:42:51 2015 Return-Path: Delivered-To: svn-src-head@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 5999860A; Sat, 24 Jan 2015 15:42:51 +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 BEF94E02; Sat, 24 Jan 2015 15:42:50 +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 t0OFgeuw051880 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Sat, 24 Jan 2015 17:42:40 +0200 (EET) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.9.2 kib.kiev.ua t0OFgeuw051880 Received: (from kostik@localhost) by tom.home (8.14.9/8.14.9/Submit) id t0OFgesv051879; Sat, 24 Jan 2015 17:42:40 +0200 (EET) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Sat, 24 Jan 2015 17:42:40 +0200 From: Konstantin Belousov To: Ian Lepore Subject: Re: svn commit: r277643 - in head/sys: arm/arm dev/mem i386/i386 mips/mips sparc64/sparc64 Message-ID: <20150124154240.GV42409@kib.kiev.ua> References: <201501241251.t0OCpGa8053192@svn.freebsd.org> <1422111397.1038.53.camel@freebsd.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1422111397.1038.53.camel@freebsd.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, svn-src-all@freebsd.org, src-committers@freebsd.org X-BeenThere: svn-src-head@freebsd.org X-Mailman-Version: 2.1.18-1 Precedence: list List-Id: SVN commit messages for the src tree for head/-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 24 Jan 2015 15:42:51 -0000 On Sat, Jan 24, 2015 at 07:56:37AM -0700, Ian Lepore wrote: > On Sat, 2015-01-24 at 12:51 +0000, Konstantin Belousov wrote: > > Author: kib > > Date: Sat Jan 24 12:51:15 2015 > > New Revision: 277643 > > URL: https://svnweb.freebsd.org/changeset/base/277643 > > > > Log: > > Remove Giant from /dev/mem and /dev/kmem. It is definitely not needed > > for i386, and from the code inspection, nothing in the > > arm/mips/sparc64 implementations depends on it. > > > > I'm not sure I agree with that. On arm the memrw() implementation uses > a single statically-allocated page of kva space into which it maps each > physical page in turn in the main loop. What prevents preemption or > multicore access to /dev/mem from trying to use that single page for > multiple operations at once? I see, thank you for noting this. But, I do not think that Giant is a solution for the problem. uiomove() call accesses userspace, which may fault and cause sleep. If the thread sleeps, the Giant is automatically dropped, so there is no real protection. I think dump exclusive sx around whole memrw() should be enough. I can revert the commit for now, or I can leave it as is while writing the patch with sx and waiting for somebody review. What would you prefer ? P.S. mips uses uiomove_fromphys(), avoiding transient mapping, and sparc allocates KVA when needed.