Re: [PATCH] Destroy the dentries contributed by a superblock on unmounting

[Andrew Morton <akpm@osdl.org>]

David Howells <dhowells@redhat.com> wrote:
>
> 
> The attached patch destroys all the dentries attached to a superblock in one go
>  /*
> + * 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);

Is the cond_resched_lock() here safe?  Once we've dropped that lock, the
list cursor `loop' is invalidated?

If all lookup paths to all entries on this list are removed at this time
then OK - but these dentries are still on the LRU..

(An answer-via-comment-patch would suit ;))