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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 CEC6AC2BD09 for ; Fri, 6 Dec 2019 16:02:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A729E24659 for ; Fri, 6 Dec 2019 16:02:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1575648165; bh=uN49J2KjPjCG2q1vrkZZhYout9vUnOgmq5AHuzL7OmY=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=CcPkMge8l/CHoJla0vH7JOCCoA0rXdaHVQxgzCXHQECoGNGo1OJ3dHjbvRVr7A0pi 57UQVzZ3mNnkjY4QLc4EsbMs/lmH9x8OfODyjKDl7YSdPw/mLjYUQCtAg8Vm3p/xFV v+a4wYnNtpTEntrNma/5Qj4iB0Do9tz5T50Fifbo= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726345AbfLFQCl (ORCPT ); Fri, 6 Dec 2019 11:02:41 -0500 Received: from mail.kernel.org ([198.145.29.99]:59266 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726258AbfLFQCk (ORCPT ); Fri, 6 Dec 2019 11:02:40 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 22B232173E; Fri, 6 Dec 2019 16:02:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1575648159; bh=uN49J2KjPjCG2q1vrkZZhYout9vUnOgmq5AHuzL7OmY=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=dhBUf+ww+91PA7OIoUovoazx+Zee9VhrqKOW7tvxd09vcuMDCb5ZqA/Xh//Y8mUTG f8gBztw7LPmmF9Ptmm0dmheqcx1kCMIhpl7Rnx+bubZ6acaSfZrRnn8cc8pKCVsolB Q0ieCCicDMxk1sWnpr3m9eetaba0Ru77BQMujAis= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id DB9F435206AB; Fri, 6 Dec 2019 08:02:38 -0800 (PST) Date: Fri, 6 Dec 2019 08:02:38 -0800 From: "Paul E. McKenney" To: madhuparnabhowmik04@gmail.com Cc: trond.myklebust@hammerspace.com, anna.schumaker@netapp.com, joel@joelfernandes.org, linux-nfs@vger.kernel.org, rcu@vger.kernel.org, linux-kernel-mentees@lists.linuxfoundation.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] fs: nfs: dir.c: Fix sparse error Message-ID: <20191206160238.GE2889@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20191206151640.10966-1-madhuparnabhowmik04@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191206151640.10966-1-madhuparnabhowmik04@gmail.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-nfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-nfs@vger.kernel.org On Fri, Dec 06, 2019 at 08:46:40PM +0530, madhuparnabhowmik04@gmail.com wrote: > From: Madhuparna Bhowmik > > This patch fixes the following errors: > fs/nfs/dir.c:2353:14: error: incompatible types in comparison expression (different address spaces): > fs/nfs/dir.c:2353:14: struct list_head [noderef] * > fs/nfs/dir.c:2353:14: struct list_head * > > caused due to directly accessing the prev pointer of > a RCU protected list. > Accessing the pointer using the macro list_prev_rcu() fixes this error. > > Signed-off-by: Madhuparna Bhowmik > --- > fs/nfs/dir.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c > index e180033e35cf..2035254cc283 100644 > --- a/fs/nfs/dir.c > +++ b/fs/nfs/dir.c > @@ -2350,7 +2350,7 @@ static int nfs_access_get_cached_rcu(struct inode *inode, const struct cred *cre > rcu_read_lock(); > if (nfsi->cache_validity & NFS_INO_INVALID_ACCESS) > goto out; > - lh = rcu_dereference(nfsi->access_cache_entry_lru.prev); > + lh = rcu_dereference(list_prev_rcu(&nfsi->access_cache_entry_lru)); And as noted in the earlier email, what is preventing concurrent insertions into and deletions from this list? o This use of list_move_tail() is OK because it does not poison. Though it isn't being all that friendly to lockless access to ->prev -- no WRITE_ONCE() in list_move_tail(). o The use of list_add_tail() is not safe with RCU readers, though they do at least partially compensate via use of smp_wmb() in nfs_access_add_cache() before calling nfs_access_add_rbtree(). o The list_del() near the end of nfs_access_add_rbtree() will poison the ->prev pointer. I don't see how this is safe given the possibility of a concurrent call to nfs_access_get_cached_rcu(). > cache = list_entry(lh, struct nfs_access_entry, lru); > if (lh == &nfsi->access_cache_entry_lru || > cred != cache->cred) And a few lines below here, it really does dereference the pointer obtained from ->prev! So how to really fix this? Here is one possibility, but we of course need to get the NFS developers' and maintainers' thoughts: o Create a list that is safe for bidirectional RCU traversal. This can use list_head, and would need these functions, give or take the exact names: list_add_tail_rcuprev(): This is like list_add_tail_rcu(), but also has smp_store_release() for ->prev. (As in there is also a __list_add_rcuprev() helper that actually contains the additional smp_store_release().) list_del_rcuprev(): This can be exactly __list_del_entry(), but with the assignment to ->prev in __list_del() becoming WRITE_ONCE(). And it looks like callers to __list_del_entry() and __list_del() might need some attention! And these might result in additional users of *_rcuprev(). list_prev_rcu() as in your first patch, but with READ_ONCE(). Otherwise DEC Alpha can fail. And more subtle compiler issues can appear on other architectures. Note that list_move_tail() will be OK give or take *_ONCE(). It might be better to define a list_move_tail_rcuprev(), given the large number of users of list_move_tail() -- some of these users might not like even the possibility of added overhead due to volatile accesses. ;-) Or am I missing something subtle here? Thanx, Paul