Thanks for you patience.
Carried out a number of basic autofs tests without problem.
So in so far as autofs4 is concerned:
On Fri, 2006-06-30 at 13:33 +0100, David Howells wrote:
> From: David Howells <dhowells@xxxxxxxxxx>
>
> The attached patch destroys all the dentries attached to a superblock in one go
> by:
>
> (1) Destroying the tree rooted at s_root.
>
> (2) Destroying every entry in the anon list, one at a time.
>
> (3) Each entry in the anon list has its subtree consumed from the leaves
> inwards.
>
> This reduces the amount of work generic_shutdown_super() does, and avoids
> iterating through the dentry_unused list.
>
> Note that locking is almost entirely absent in the shrink_dcache_for_umount*()
> functions added by this patch. This is because:
>
> (1) at the point the filesystem calls generic_shutdown_super(), it is not
> permitted to further touch the superblock's set of dentries, and nor may
> it remove aliases from inodes;
>
> (2) the dcache memory shrinker now skips dentries that are being unmounted;
> and
>
> (3) the superblock no longer has any external references through which the VFS
> can reach it.
>
> Given these points, the only locking we need to do is when we remove dentries
> from the unused list and the name hashes, which we do a directory's worth at a
> time.
>
> We also don't need to guard against reference counts going to zero unexpectedly
> and removing bits of the tree we're working on as nothing else can call dput().
>
> A cut down version of dentry_iput() has been folded into
> shrink_dcache_for_umount_subtree() function. Apart from not needing to unlock
> things, it also doesn't need to check for inotify watches.
>
> In this version of the patch, the complaint about a dentry still being in use
> has been expanded from a single BUG_ON() and now gives much more information.
>
> Signed-Off-By: David Howells <dhowells@xxxxxxxxxx>
> Acked-by: NeilBrown <neilb@xxxxxxx>
Acked-by: Ian Kent <raven@xxxxxxxxxx>
> ---
>
> fs/dcache.c | 133 ++++++++++++++++++++++++++++++++++++++++++++++++
> fs/super.c | 12 ++--
> include/linux/dcache.h | 1
> 3 files changed, 140 insertions(+), 6 deletions(-)
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 48b44a7..ae041da 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -548,6 +548,139 @@ repeat:
> }
>
> /*
> + * destroy a single subtree of dentries for unmount
> + * - see the comments on shrink_dcache_for_umount() for a description of the
> + * locking
> + */
> +static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
> +{
> + struct dentry *parent;
> +
> + BUG_ON(!IS_ROOT(dentry));
> +
> + /* detach this root from the system */
> + spin_lock(&dcache_lock);
> + if (!list_empty(&dentry->d_lru)) {
> + dentry_stat.nr_unused--;
> + list_del_init(&dentry->d_lru);
> + }
> + __d_drop(dentry);
> + spin_unlock(&dcache_lock);
> +
> + for (;;) {
> + /* descend to the first leaf in the current subtree */
> + while (!list_empty(&dentry->d_subdirs)) {
> + struct dentry *loop;
> +
> + /* this is a branch with children - detach all of them
> + * from the system in one go */
> + spin_lock(&dcache_lock);
> + list_for_each_entry(loop, &dentry->d_subdirs,
> + d_u.d_child) {
> + if (!list_empty(&loop->d_lru)) {
> + dentry_stat.nr_unused--;
> + list_del_init(&loop->d_lru);
> + }
> +
> + __d_drop(loop);
> + cond_resched_lock(&dcache_lock);
> + }
> + spin_unlock(&dcache_lock);
> +
> + /* move to the first child */
> + dentry = list_entry(dentry->d_subdirs.next,
> + struct dentry, d_u.d_child);
> + }
> +
> + /* consume the dentries from this leaf up through its parents
> + * until we find one with children or run out altogether */
> + do {
> + struct inode *inode;
> +
> + if (atomic_read(&dentry->d_count) != 0) {
> + printk(KERN_ERR
> + "BUG: Dentry %p{i=%lx,n=%s}"
> + " still in use (%d)"
> + " [unmount of %s %s]\n",
> + dentry,
> + dentry->d_inode ?
> + dentry->d_inode->i_ino : 0UL,
> + dentry->d_name.name,
> + atomic_read(&dentry->d_count),
> + dentry->d_sb->s_type->name,
> + dentry->d_sb->s_id);
> + BUG();
> + }
> +
> + parent = dentry->d_parent;
> + if (parent == dentry)
> + parent = NULL;
> + else
> + atomic_dec(&parent->d_count);
> +
> + list_del(&dentry->d_u.d_child);
> + dentry_stat.nr_dentry--; /* For d_free, below */
> +
> + inode = dentry->d_inode;
> + if (inode) {
> +#ifdef CONFIG_INOTIFY
> + BUG_ON(!list_empty(&inode->inotify_watches));
> +#endif
> + dentry->d_inode = NULL;
> + list_del_init(&dentry->d_alias);
> + if (dentry->d_op && dentry->d_op->d_iput)
> + dentry->d_op->d_iput(dentry, inode);
> + else
> + iput(inode);
> + }
> +
> + d_free(dentry);
> +
> + /* finished when we fall off the top of the tree,
> + * otherwise we ascend to the parent and move to the
> + * next sibling if there is one */
> + if (!parent)
> + return;
> +
> + dentry = parent;
> +
> + } while (list_empty(&dentry->d_subdirs));
> +
> + dentry = list_entry(dentry->d_subdirs.next,
> + struct dentry, d_u.d_child);
> + }
> +}
> +
> +/*
> + * destroy the dentries attached to a superblock on unmounting
> + * - we don't need to use dentry->d_lock, and only need dcache_lock when
> + * removing the dentry from the system lists and hashes because:
> + * - the superblock is detached from all mountings and open files, so the
> + * dentry trees will not be rearranged by the VFS
> + * - s_umount is write-locked, so the memory pressure shrinker will ignore
> + * any dentries belonging to this superblock that it comes across
> + * - the filesystem itself is no longer permitted to rearrange the dentries
> + * in this superblock
> + */
> +void shrink_dcache_for_umount(struct super_block *sb)
> +{
> + struct dentry *dentry;
> +
> + if (down_read_trylock(&sb->s_umount))
> + BUG();
> +
> + dentry = sb->s_root;
> + sb->s_root = NULL;
> + atomic_dec(&dentry->d_count);
> + shrink_dcache_for_umount_subtree(dentry);
> +
> + while (!hlist_empty(&sb->s_anon)) {
> + dentry = hlist_entry(sb->s_anon.first, struct dentry, d_hash);
> + shrink_dcache_for_umount_subtree(dentry);
> + }
> +}
> +
> +/*
> * Search for at least 1 mount point in the dentry's subdirs.
> * We descend to the next level whenever the d_subdirs
> * list is non-empty and continue searching.
> diff --git a/fs/super.c b/fs/super.c
> index 8a669f6..bd655b1 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -222,17 +222,17 @@ static int grab_super(struct super_block
> * that need destruction out of superblock, call generic_shutdown_super()
> * and release aforementioned objects. Note: dentries and inodes _are_
> * taken care of and do not need specific handling.
> + *
> + * Upon calling this function, the filesystem may no longer alter or
> + * rearrange the set of dentries belonging to this super_block, nor may it
> + * change the attachments of dentries to inodes.
> */
> void generic_shutdown_super(struct super_block *sb)
> {
> - struct dentry *root = sb->s_root;
> struct super_operations *sop = sb->s_op;
>
> - if (root) {
> - sb->s_root = NULL;
> - shrink_dcache_parent(root);
> - shrink_dcache_sb(sb);
> - dput(root);
> + if (sb->s_root) {
> + shrink_dcache_for_umount(sb);
> fsync_super(sb);
> lock_super(sb);
> sb->s_flags &= ~MS_ACTIVE;
> diff --git a/include/linux/dcache.h b/include/linux/dcache.h
> index 0dd1610..fe8bcd5 100644
> --- a/include/linux/dcache.h
> +++ b/include/linux/dcache.h
> @@ -217,6 +217,7 @@ extern struct dentry * d_alloc_anon(stru
> extern struct dentry * d_splice_alias(struct inode *, struct dentry *);
> extern void shrink_dcache_sb(struct super_block *);
> extern void shrink_dcache_parent(struct dentry *);
> +extern void shrink_dcache_for_umount(struct super_block *);
> extern int d_invalidate(struct dentry *);
>
> /* only used at mount-time */
-
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