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Fri, 4 Sep 2026 17:53:38 -0400 (EDT) From: NeilBrown To: Alexander Viro , Christian Brauner Cc: Jan Kara , linux-fsdevel@vger.kernel.org, Jeff Layton , Amir Goldstein , Miklos Szeredi , linux-kernel@vger.kernel.org Subject: [PATCH v4 4/7] VFS: add d_duplicate() Date: Sat, 5 Sep 2026 07:48:13 +1000 Message-ID: <20260904215142.1060510-5-neilb@ownmail.net> X-Mailer: git-send-email 2.50.0.107.gf914562f5916.dirty In-Reply-To: <20260904215142.1060510-1-neilb@ownmail.net> References: <20260904215142.1060510-1-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 43149c7849d9..cbd5738de168 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -2000,6 +2000,57 @@ 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 the old MUST either be moved + * or dropped. + * + * Parent must be locked. + * + * Returns: an in-lookup dentry, or -ENOMEM. + */ +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(&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 7afe16d4664d..2b7d99ec9306 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_trylock(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