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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 EF774C48BE8 for ; Sun, 13 Jun 2021 01:46:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B1CA460FEA for ; Sun, 13 Jun 2021 01:46:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231537AbhFMBdi (ORCPT ); Sat, 12 Jun 2021 21:33:38 -0400 Received: from wnew1-smtp.messagingengine.com ([64.147.123.26]:60375 "EHLO wnew1-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229753AbhFMBdh (ORCPT ); Sat, 12 Jun 2021 21:33:37 -0400 Received: from compute2.internal (compute2.nyi.internal [10.202.2.42]) by mailnew.west.internal (Postfix) with ESMTP id 7E07A10D2; Sat, 12 Jun 2021 21:31:36 -0400 (EDT) Received: from mailfrontend2 ([10.202.2.163]) by compute2.internal (MEProxy); Sat, 12 Jun 2021 21:31:37 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=themaw.net; h= message-id:subject:from:to:cc:date:in-reply-to:references :content-type:mime-version:content-transfer-encoding; s=fm3; bh= b3t8hZRfSlI/Y+tiwVm5+qLUp7/kF3OUNDPX0rpg0OU=; b=MiGQ5fKSbeUntkY1 BqYrvQwca864HsTIeGdrzvMe06t4A9aGjI2yenNWmcwgZpmGutMsg+TJ6LOvy3er Ixwqxjb62KmKvmwaAM1lq3LV4hc017odWeLRhi3a2yjPCNc3WyBo56h12bGAa8DV w3gvAdSLjjrAdjS1o+Gg8znQQ9bLvN/F20XMl2VuRiOcQVyJ+nwhBM0qmkG7YTdy BC3rAwbz1JW0s1qA7gyNekCWs+SwchgXWNpCyp+D9tFR6z25Gn/bbCjk0RQPPiPy S9Fkpi4HBuGI7p7pP+urm/4JK+EjKRueimvluXLYk2sThXYU9tL62OvwlB6HO+mt AyHdow== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d= messagingengine.com; h=cc:content-transfer-encoding:content-type :date:from:in-reply-to:message-id:mime-version:references :subject:to:x-me-proxy:x-me-proxy:x-me-sender:x-me-sender :x-sasl-enc; s=fm3; bh=b3t8hZRfSlI/Y+tiwVm5+qLUp7/kF3OUNDPX0rpg0 OU=; b=eOZ0el+P2R1qoPaam9vQWrtqgEHXyVhLkpwPnLDO4N3o6PH9X3q5/7jJ+ U9iqDrxTf2dXItDAn/hOY7GL0rUZzh/flFDI1yuS82WOhaykBfpVnuvDZG2LR5Dr FyF1z4teMFYCD2TKjJTEXZs0UI0AT5fcIKacbKWwbXW1NUtx2GZBCGebc6iCgmbF kYd5E6C2E6H6hC2d3L2mR9Peytk03Va+CrbY5h9Nc/2qaM/UJYrwXlsxqCqp7lLb AKJJDT6J+jEnQjq7whiYdn4ipMXlXOvTFxRzssZUb6gNHRsmqFxgRVMkwnKZnYoK saj9u0uhDON8GZWKjOYGaYzJJEvhw== X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeduledrfedvuddggedvucetufdoteggodetrfdotf fvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfqfgfvpdfurfetoffkrfgpnffqhgen uceurghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmne cujfgurhepkffuhffvffgjfhgtfggggfesthekredttderjeenucfhrhhomhepkfgrnhcu mfgvnhhtuceorhgrvhgvnhesthhhvghmrgifrdhnvghtqeenucggtffrrghtthgvrhhnpe fgleelkeetheelgeehueejueduhfeufffgleehgfevtdehhffhhffhtddugfefheenucev lhhushhtvghrufhiiigvpedtnecurfgrrhgrmhepmhgrihhlfhhrohhmpehrrghvvghnse hthhgvmhgrfidrnhgvth X-ME-Proxy: Received: by mail.messagingengine.com (Postfix) with ESMTPA; Sat, 12 Jun 2021 21:31:30 -0400 (EDT) Message-ID: <1d6b99854f6c3d7948882d745b3fef9a3116ab73.camel@themaw.net> Subject: Re: [PATCH v6 5/7] kernfs: use i_lock to protect concurrent inode updates From: Ian Kent To: Al Viro Cc: Greg Kroah-Hartman , Tejun Heo , Eric Sandeen , Fox Chen , Brice Goglin , Rick Lindsley , David Howells , Miklos Szeredi , Marcelo Tosatti , "Eric W. Biederman" , Carlos Maiolino , linux-fsdevel , Kernel Mailing List Date: Sun, 13 Jun 2021 09:31:26 +0800 In-Reply-To: References: <162322846765.361452.17051755721944717990.stgit@web.messagingengine.com> <162322868275.361452.17585267026652222121.stgit@web.messagingengine.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.38.4 (3.38.4-1.fc33) MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Sat, 2021-06-12 at 01:45 +0000, Al Viro wrote: > On Wed, Jun 09, 2021 at 04:51:22PM +0800, Ian Kent wrote: > > The inode operations .permission() and .getattr() use the kernfs > > node > > write lock but all that's needed is to keep the rb tree stable > > while > > updating the inode attributes as well as protecting the update > > itself > > against concurrent changes. > > Huh?  Where does it access the rbtree at all?  Confused... That description's wrong, I'll fix that. > > > diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c > > index 3b01e9e61f14e..6728ecd81eb37 100644 > > --- a/fs/kernfs/inode.c > > +++ b/fs/kernfs/inode.c > > @@ -172,6 +172,7 @@ static void kernfs_refresh_inode(struct > > kernfs_node *kn, struct inode *inode) > >  { > >         struct kernfs_iattrs *attrs = kn->iattr; > >   > > +       spin_lock(&inode->i_lock); > >         inode->i_mode = kn->mode; > >         if (attrs) > >                 /* > > @@ -182,6 +183,7 @@ static void kernfs_refresh_inode(struct > > kernfs_node *kn, struct inode *inode) > >   > >         if (kernfs_type(kn) == KERNFS_DIR) > >                 set_nlink(inode, kn->dir.subdirs + 2); > > +       spin_unlock(&inode->i_lock); > >  } > > Even more so - just what are you serializing here?  That code > synchronizes inode > metadata with those in kernfs_node.  Suppose you've got two threads > doing > ->permission(); the first one gets through kernfs_refresh_inode() and > goes into > generic_permission().  No locks are held, so kernfs_refresh_inode() > from another > thread can run in parallel with generic_permission(). > > If that's not a problem, why two kernfs_refresh_inode() done in > parallel would > be a problem? > > Thread 1: >         permission >                 done refresh, all locks released now > Thread 2: >         change metadata in kernfs_node > Thread 2: >         permission >                 goes into refresh, copying metadata into inode > Thread 1: >                 generic_permission() > No locks in common between the last two operations, so > we generic_permission() might see partially updated metadata. > Either we don't give a fuck (in which case I don't understand > what purpose does that ->i_lock serve) *or* we need the exclusion > to cover a wider area.