[Devel] Re: [Q] missing unused dentry in prune_dcache()?

[Vasily Averin]

David Howells wrote:
> Vasily Averin <vvs at sw.ru> wrote:
> 
>> I've noticed one more minor issue in your patch: in
>> shrink_dcache_for_umount_subtree() function you decrement
>> dentry_stat.nr_dentry without dcache_lock.
> 
> How about the attached patch?

I would note that in first two hunks you have decremented dentry_stat.nr_unused
correctly, under dcache_lock. Probably it's better to count freed dentries only
in third case and corrects dentry_stat.nr_unused value inside
shrink_dcache_for_umount_subtree() function before return.

Thank you,
	Vasily Averin
> David
> ---
> VFS: Fix an error in unused dentry counting
> 
> From: David Howells <dhowells at redhat.com>
> 
> Fix an error in unused dentry counting in shrink_dcache_for_umount_subtree() in
> which the count is modified without the dcache_lock held.
> 
> Signed-Off-By: David Howells <dhowells at redhat.com>
> ---
> 
>  fs/dcache.c |   22 +++++++++++++++-------
>  1 files changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index a1ff91e..eab1bf4 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -553,16 +553,17 @@ repeat:
>   * - see the comments on shrink_dcache_for_umount() for a description of the
>   *   locking
>   */
> -static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
> +static unsigned shrink_dcache_for_umount_subtree(struct dentry *dentry)
>  {
>  	struct dentry *parent;
> +	unsigned detached = 0;
>  
>  	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--;
> +		detached++;
>  		list_del_init(&dentry->d_lru);
>  	}
>  	__d_drop(dentry);
> @@ -579,7 +580,7 @@ static void shrink_dcache_for_umount_sub
>  			list_for_each_entry(loop, &dentry->d_subdirs,
>  					    d_u.d_child) {
>  				if (!list_empty(&loop->d_lru)) {
> -					dentry_stat.nr_unused--;
> +					detached++;
>  					list_del_init(&loop->d_lru);
>  				}
>  
> @@ -620,7 +621,7 @@ static void shrink_dcache_for_umount_sub
>  				atomic_dec(&parent->d_count);
>  
>  			list_del(&dentry->d_u.d_child);
> -			dentry_stat.nr_dentry--;	/* For d_free, below */
> +			detached++;	/* For d_free, below */
>  
>  			inode = dentry->d_inode;
>  			if (inode) {
> @@ -638,7 +639,7 @@ static void shrink_dcache_for_umount_sub
>  			 * otherwise we ascend to the parent and move to the
>  			 * next sibling if there is one */
>  			if (!parent)
> -				return;
> +				return detached;
>  
>  			dentry = parent;
>  
> @@ -663,6 +664,7 @@ static void shrink_dcache_for_umount_sub
>  void shrink_dcache_for_umount(struct super_block *sb)
>  {
>  	struct dentry *dentry;
> +	unsigned detached = 0;
>  
>  	if (down_read_trylock(&sb->s_umount))
>  		BUG();
> @@ -670,11 +672,17 @@ void shrink_dcache_for_umount(struct sup
>  	dentry = sb->s_root;
>  	sb->s_root = NULL;
>  	atomic_dec(&dentry->d_count);
> -	shrink_dcache_for_umount_subtree(dentry);
> +	detached = 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);
> +		detached += shrink_dcache_for_umount_subtree(dentry);
> +	}
> +
> +	if (detached) {
> +		spin_lock(&dcache_lock);
> +		dentry_stat.nr_unused -= detached;
> +		spin_unlock(&dcache_lock);
>  	}
>  }
>  
>