From: NeilBrown <[email protected]> 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 target 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 <[email protected]> --- fs/dcache.c | 52 ++++++++++++++++++++++++++++++++++++++++++ include/linux/dcache.h | 1 + 2 files changed, 53 insertions(+) diff --git a/fs/dcache.c b/fs/dcache.c index f4d7d200bc46..c12319097d6e 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -1832,6 +1832,58 @@ 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, os 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 and the 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; + new->d_wait = NULL; + 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_u.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 3cb70b3398f0..2a3ebd368ed9 100644 --- a/include/linux/dcache.h +++ b/include/linux/dcache.h @@ -247,6 +247,7 @@ 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
