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 X-Spam-Level: X-Spam-Status: No, score=-2.8 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED, USER_AGENT_NEOMUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 53070C10F14 for ; Mon, 8 Apr 2019 22:21:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 204C421473 for ; Mon, 8 Apr 2019 22:21:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=armlinux.org.uk header.i=@armlinux.org.uk header.b="bXd/J4j1" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726938AbfDHWVY (ORCPT ); Mon, 8 Apr 2019 18:21:24 -0400 Received: from pandora.armlinux.org.uk ([78.32.30.218]:35840 "EHLO pandora.armlinux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726188AbfDHWVY (ORCPT ); Mon, 8 Apr 2019 18:21:24 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=armlinux.org.uk; s=pandora-2019; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=9404zG5Mey352Oli4qhYASTVbszYTAXe3u6LsYDlhXU=; b=bXd/J4j1dkOVh6Ru9bRKKsTnK 9wWbBzq4pNDwdaka/5pF+ylB0SWFQilNKPMPJl4+WJ9W7vjv8VCe+sZN/RQYtnMUBva9TRWfGOkej U2y569cQOmsCoe3Q9C/zlAXPkB9GwjkkJa0f5svnZBwuMUQRaFJ/8PHsE/RXD3C8ABr9Lb1p64di+ vSODMYC1uLuzrHm7RQKLDj47xz+MO0F6/AY4nL7lsdhnCh0AbOuno+n84pO0S7ScYEjwzX4RVHpJH dGIKK0i1XI+ng7g1Z75MRye4oA8IAy8GY4SnHxqWL8y1J9FwERAeaqRffFdxm5SMmng51cEjXF37b R3BNc6ZFw==; Received: from shell.armlinux.org.uk ([fd8f:7570:feb6:1:5054:ff:fe00:4ec]:52022) by pandora.armlinux.org.uk with esmtpsa (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.90_1) (envelope-from ) id 1hDcdb-0001ms-A5; Mon, 08 Apr 2019 23:21:15 +0100 Received: from linux by shell.armlinux.org.uk with local (Exim 4.89) (envelope-from ) id 1hDcdW-0001Ln-K8; Mon, 08 Apr 2019 23:21:06 +0100 Date: Mon, 8 Apr 2019 23:21:06 +0100 From: Russell King - ARM Linux admin To: Stephen Boyd Cc: Matti Vaittinen , "mturquette@baylibre.com" , "linux-kernel@vger.kernel.org" , "wens@csie.org" , "miquel.raynal@bootlin.com" , "jhugo@codeaurora.org" , "linux-clk@vger.kernel.org" , "jbrunet@baylibre.com" Subject: Re: [PATCH v3 1/7] clkdev: Hold clocks_mutex while iterating clocks list Message-ID: <20190408222106.ect7nwjsklzhmdwj@shell.armlinux.org.uk> References: <20190404215344.6330-1-sboyd@kernel.org> <20190404215344.6330-2-sboyd@kernel.org> <155449664462.20095.10904772826291270300@swboyd.mtv.corp.google.com> <20190408104941.GB17594@localhost.localdomain> <155474280295.20095.182612336816713159@swboyd.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <155474280295.20095.182612336816713159@swboyd.mtv.corp.google.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 08, 2019 at 10:00:02AM -0700, Stephen Boyd wrote: > Quoting Matti Vaittinen (2019-04-08 03:49:41) > > On Fri, Apr 05, 2019 at 01:37:24PM -0700, Stephen Boyd wrote: > > > Quoting Vaittinen, Matti (2019-04-04 23:51:43) > > > > On Thu, 2019-04-04 at 14:53 -0700, Stephen Boyd wrote: > > > > > We recently introduced a change to support devm clk lookups. That > > > > > change > > > > > introduced a code-path that used clk_find() without holding the > > > > > 'clocks_mutex'. Unfortunately, clk_find() iterates over the 'clocks' > > > > > list and so we need to prevent the list from being modified while > > > > > iterating over it by holding the mutex. Similarly, we don't need to > > > > > hold > > > > > the 'clocks_mutex' besides when we're dereferencing the clk_lookup > > > > > pointer > > > > > > > > /// Snip > > > > > > > > > -out: > > > > > +static struct clk_lookup *clk_find(const char *dev_id, const char > > > > > *con_id) > > > > > +{ > > > > > + struct clk_lookup *cl; > > > > > + > > > > > + mutex_lock(&clocks_mutex); > > > > > + cl = __clk_find(dev_id, con_id); > > > > > mutex_unlock(&clocks_mutex); > > > > > > > > > > - return cl ? clk : ERR_PTR(-ENOENT); > > > > > + return cl; > > > > > +} > > > > > > > > I am not an expert on this but reading commit message abowe and seeing > > > > the code for clk_find() looks a bit scary. If I understand it > > > > correctly, the clocks_mutex should be held when dereferencing the > > > > clk_lookup returned by clk_find. The clk_find implementation drops the > > > > lock before returning - which makes me think I miss something here. How > > > > can the caller ever safely dereference returned clk_lookup pointer? > > > > Just reading abowe makes me think that lock should be taken by whoever > > > > is calling the clk_find, and dropped only after caller has used the > > > > found clk_lookup for whatever caller intends to use it. Maybe I am > > > > missing something? > > > > > > > > > > The only user after this patch (devm) is doing a pointer comparison so > > > it looks OK. But yes, in general there shouldn't be users of clk_find() > > > that dereference the pointer because there isn't any protection besides > > > the mutex. > > > > If the only (intended) user for clk_find is devm_clk_release_clkdev, > > then we might want to write it in devm_clk_release_clkdev - just to > > avoid similar errors (as I did with devm) in the future? I might even > > consider renaming __clk_find to clk_find or to clk_find_unsafe - but > > that's all just nitpicking :) Go with what you like to maintain :D > > > > Sure. I was thinking along the same lines after you asked. This is rubbish. The reason clk_find() doesn't take the lock is that you _need_ to hold the lock while you dereference the clk_lookup data. The lock isn't protecting just the lookup, it protects what you do with the result of the lookup as well. So, as I say, adding locking inside clk_find() is completely misunderstanding the locking here. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up