From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from fout-b7-smtp.messagingengine.com (fout-b7-smtp.messagingengine.com [202.12.124.150]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 556E03451C1; Thu, 30 Apr 2026 03:09:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=202.12.124.150 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777518579; cv=none; b=gjg4jYb1rtIUJNQ5Rv9YfuuB4I5eU1QxIwoCieM87qOJMJFVYc1GIuArY4m2iv0eSDl4v8yp4KhvdZ3zslqMTQ2NBU+tyK2gp0Vn0JU9+GOI58x7d9GhvSaooaacD3HjzyiaeKboRGmgZMikeXDQvcwKaxXIUhcLhYYFO75fIhE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777518579; c=relaxed/simple; bh=aI/7LxQasa/WMqtdcaEEHsmvZknfOqE36vo7BSVAHxc=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=t9usimgavFeJSts7ePwW6BGVFgjzA68dDSKG9+pMbHBUH2STbqL9BWuvY/UbqnrCWumwE8nK6zH64wW4t+XEwCDhNkDTAQE7T5naZ+2LImb8CUVdo6pPrV6h1BDA2n+6/gqnwSwbJL3b5BXWDvMbWFAweUP5xmGCT9KrTUhd0ZE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=ownmail.net; spf=pass smtp.mailfrom=ownmail.net; dkim=pass (2048-bit key) header.d=ownmail.net header.i=@ownmail.net header.b=jiFKN5H6; dkim=pass (2048-bit key) header.d=messagingengine.com header.i=@messagingengine.com header.b=WIHFdhxp; arc=none smtp.client-ip=202.12.124.150 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=ownmail.net Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=ownmail.net Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=ownmail.net header.i=@ownmail.net header.b="jiFKN5H6"; dkim=pass (2048-bit key) header.d=messagingengine.com header.i=@messagingengine.com header.b="WIHFdhxp" Received: from phl-compute-03.internal (phl-compute-03.internal [10.202.2.43]) by mailfout.stl.internal (Postfix) with ESMTP id 529C81D00113; Wed, 29 Apr 2026 23:09:37 -0400 (EDT) Received: from phl-frontend-04 ([10.202.2.163]) by phl-compute-03.internal (MEProxy); Wed, 29 Apr 2026 23:09:37 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ownmail.net; h= cc:cc:content-transfer-encoding:content-type:date:date:from:from :in-reply-to:in-reply-to:message-id:mime-version:references :reply-to:reply-to:subject:subject:to:to; s=fm2; t=1777518577; x=1777604977; bh=NvBayTPXi0DVU1I5ML5QY0CEm4bJY9EzhHJqYyVRowQ=; b= jiFKN5H6lMe21XIRD5gxVqYnUexildLkn+nn77y2172uxtAJkftIPBJkpSnU/Bnb qPbUxEtrnV3U1Afxc64o10VFDLAlxREpp3uvF0SaZiYWeJeUMutViCUzn9Y/ZA79 mZ2ubLD5DOwuI8xnfccYBw5/E1jEXsXepwc4ZGK54cmVj+3Hwrb3q1wIAcZ9ruNz IJN6va1f5NPPsfxVF3QRSF+d8VBy+eaN+mpq7y3RO0V96kxbDhk/P7tcaKxD8DPc o2EHZseWL5Z7nT9DD9X2a3KkdP8f/8953jMQULmeivRDWDMPc0LZZ4trbZkQEdKW YXi8exxHJ+kAzHf/NU5apg== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d= messagingengine.com; h=cc:cc:content-transfer-encoding :content-type:date:date:feedback-id:feedback-id:from:from :in-reply-to:in-reply-to:message-id:mime-version:references :reply-to:reply-to:subject:subject:to:to:x-me-proxy:x-me-sender :x-me-sender:x-sasl-enc; s=fm2; t=1777518577; x=1777604977; bh=N vBayTPXi0DVU1I5ML5QY0CEm4bJY9EzhHJqYyVRowQ=; b=WIHFdhxpuK8pVZ88X r+XSaTC8kpZv89AXB56QmwnRIWdKd63b2n8obrEMCXVuz0FxoXzSQiPV+Jr34X8u Olpx/nXRtHjGowxlP0ek1q3JIaOBQzApSLy0ayyG3g3m9fdRNcZsbHydr0JOuGR+ tPQAzMg0ndhHYKRJ+dewMrs9fDxwLquHjJ+fmxSzz+tO8iCP5rPi8waerXb6Q9Mh UdfwKAQxegZLxYK+VnOwqxWBE5eQGjFvJIMP1LC+UETIAzHpZ0aoaXe9gusNwfs3 HAqnoN0RiLm7MKXZyLcggUxMaFLMMRc3X7kUcGH0EUYba7E/IPVgrF/E4Uj8irrP FgWEw== X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeefhedrtddtgdekiedvtdcutefuodetggdotefrod ftvfcurfhrohhfihhlvgemucfhrghsthforghilhdpuffrtefokffrpgfnqfghnecuuegr ihhlohhuthemuceftddtnecusecvtfgvtghiphhivghnthhsucdlqddutddtmdenucfjug hrpefhvfevufffkffojghfrhgggfestdekredtredttdenucfhrhhomheppfgvihhluehr ohifnhcuoehnvghilhgssehofihnmhgrihhlrdhnvghtqeenucggtffrrghtthgvrhhnpe evveekffduueevhfeigefhgfdukedtleekjeeitdejudfgueekvdekffdvfedvudenucev lhhushhtvghrufhiiigvpedtnecurfgrrhgrmhepmhgrihhlfhhrohhmpehnvghilhgsse hofihnmhgrihhlrdhnvghtpdhnsggprhgtphhtthhopeekpdhmohguvgepshhmthhpohhu thdprhgtphhtthhopehvihhrohesiigvnhhivhdrlhhinhhugidrohhrghdruhhkpdhrtg hpthhtoheplhhinhhugidqkhgvrhhnvghlsehvghgvrhdrkhgvrhhnvghlrdhorhhgpdhr tghpthhtoheplhhinhhugidqfhhsuggvvhgvlhesvhhgvghrrdhkvghrnhgvlhdrohhrgh dprhgtphhtthhopehjrggtkhesshhushgvrdgtiidprhgtphhtthhopehtohhrvhgrlhgu sheslhhinhhugidqfhhouhhnuggrthhiohhnrdhorhhgpdhrtghpthhtohepjhhlrgihth honheskhgvrhhnvghlrdhorhhgpdhrtghpthhtohepsghrrghunhgvrheskhgvrhhnvghl rdhorhhgpdhrtghpthhtoheprghmihhrjeefihhlsehgmhgrihhlrdgtohhm X-ME-Proxy: Feedback-ID: i9d664b8f:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Wed, 29 Apr 2026 23:09:34 -0400 (EDT) From: NeilBrown To: Linus Torvalds , Al Viro , Christian Brauner , Jan Kara , Jeff Layton , Amir Goldstein Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v4 5/7] VFS: add d_duplicate() Date: Thu, 30 Apr 2026 11:56:58 +1000 Message-ID: <20260430020137.3305302-6-neilb@ownmail.net> X-Mailer: git-send-email 2.50.0.107.gf914562f5916.dirty In-Reply-To: <20260430020137.3305302-1-neilb@ownmail.net> References: <20260430020137.3305302-1-neilb@ownmail.net> Reply-To: NeilBrown Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: NeilBrown Occasionally a single operation can require two sub-operations on the same name, and it is important that a d_alloc_parallel() (once that can be run unlocked) does not create another dentry with the same name between the operations. Two examples: 1/ rename where the target name (a positive dentry) needs to be "silly-renamed" to a temporary name so it will remain available on the server (NFS and AFS). Here the same name needs to be the subject of one rename, and the target of another. 2/ rename where the subject needs to be replaced with a white-out (shmemfs). Here the same name need to be the subject of a rename and the target of a mknod() In both cases the original dentry is renamed to something else, and a replacement is instantiated, possibly as the target of d_move(), possibly by d_instantiate(). Currently d_alloc() is used to create the dentry and the exclusive lock on the parent ensures no other dentry is created. When d_alloc_parallel() is moved out of the parent lock, this will no longer be sufficient. In particular if the original is renamed away before the new is instantiated, there is a window where d_alloc_parallel() could create another name. "silly-rename" does work in this order. shmemfs whiteout doesn't open this hole but is essentially the same pattern and should use the same approach. The new d_duplicate() creates an in-lookup dentry with the same name as the original dentry, which must be hashed. There is no need to check if an in-lookup dentry exists with the same name as d_alloc_parallel() will never try add one while the hashed dentry exists. Once the new in-lookup is created, d_alloc_parallel() will find it and wait for it to complete, then use it. Signed-off-by: NeilBrown --- fs/dcache.c | 51 ++++++++++++++++++++++++++++++++++++++++++ include/linux/dcache.h | 1 + 2 files changed, 52 insertions(+) diff --git a/fs/dcache.c b/fs/dcache.c index a0096ac02790..1943607f7547 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -1900,6 +1900,57 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name) } EXPORT_SYMBOL(d_alloc); +/** + * d_duplicate - duplicate a dentry for combined atomic operation + * @dentry: the dentry to duplicate + * + * Some rename operations need to be combined with another operation + * inside the filesystem. + * 1/ A cluster filesystem when renaming to an in-use file might need to + * first "silly-rename" that target out of the way before the main rename + * 2/ A filesystem that supports white-out might want to create a whiteout + * in place of the file being moved. + * + * For this they need two dentries which temporarily have the same name, + * before one is renamed. d_duplicate() provides for this. Given a + * positive hashed dentry, it creates a second in-lookup dentry. + * Because the original dentry exists, no other thread will try to + * create an in-lookup dentry, so there can be no race in this create. + * + * The caller should d_move() the original to a new name, often via a + * rename request, and should call d_lookup_done() on the newly created + * dentry. If the new is instantiated then old MUST either be moved + * or dropped. + * + * Parent must be locked. + * + * Returns: an in-lookup dentry, or an error. + */ +struct dentry *d_duplicate(struct dentry *dentry) +{ + unsigned int hash = dentry->d_name.hash; + struct dentry *parent = dentry->d_parent; + struct hlist_bl_head *b = in_lookup_hash(parent, hash); + struct dentry *new = __d_alloc(parent->d_sb, &dentry->d_name); + + if (unlikely(!new)) + return ERR_PTR(-ENOMEM); + + new->d_flags |= DCACHE_PAR_LOOKUP; + spin_lock(&parent->d_lock); + new->d_parent = dget_dlock(parent); + hlist_add_head(&new->d_sib, &parent->d_children); + if (parent->d_flags & DCACHE_DISCONNECTED) + new->d_flags |= DCACHE_DISCONNECTED; + spin_unlock(&dentry->d_parent->d_lock); + + hlist_bl_lock(b); + hlist_bl_add_head(&new->d_in_lookup_hash, b); + hlist_bl_unlock(b); + return new; +} +EXPORT_SYMBOL(d_duplicate); + struct dentry *d_alloc_anon(struct super_block *sb) { return __d_alloc(sb, NULL); diff --git a/include/linux/dcache.h b/include/linux/dcache.h index d22c308043bb..b4663a1a0636 100644 --- a/include/linux/dcache.h +++ b/include/linux/dcache.h @@ -259,6 +259,7 @@ extern struct dentry * d_alloc_anon(struct super_block *); extern struct dentry * d_alloc_parallel(struct dentry *, const struct qstr *); extern struct dentry * d_alloc_noblock(struct dentry *, struct qstr *); extern struct dentry * d_splice_alias(struct inode *, struct dentry *); +struct dentry *d_duplicate(struct dentry *dentry); /* weird procfs mess; *NOT* exported */ extern struct dentry * d_splice_alias_ops(struct inode *, struct dentry *, const struct dentry_operations *); -- 2.50.0.107.gf914562f5916.dirty