Re: [PATCH v2 1/6] libfs: Re-arrange locking in offset_iterate_dir()

From: Jan Kara <jack@suse.cz>
To: Chuck Lever <cel@kernel.org>
Cc: viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz,
	hughd@google.com, akpm@linux-foundation.org,
	Liam.Howlett@oracle.com, oliver.sang@intel.com,
	feng.tang@intel.com, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, maple-tree@lists.infradead.org,
	linux-mm@kvack.org, lkp@intel.com
Subject: Re: [PATCH v2 1/6] libfs: Re-arrange locking in offset_iterate_dir()
Date: Wed, 21 Feb 2024 14:15:39 +0100	[thread overview]
Message-ID: <20240221131539.wmr5xde7xode2xn5@quack3> (raw)
In-Reply-To: <170820142021.6328.15047865406275957018.stgit@91.116.238.104.host.secureserver.net>

On Sat 17-02-24 15:23:40, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> Liam and Matthew say that once the RCU read lock is released,
> xa_state is not safe to re-use for the next xas_find() call. But the
> RCU read lock must be released on each loop iteration so that
> dput(), which might_sleep(), can be called safely.
> 
> Thus we are forced to walk the offset tree with fresh state for each
> directory entry. xa_find() can do this for us, though it might be a
> little less efficient than maintaining xa_state locally.
> 
> We believe that in the current code base, inode->i_rwsem provides
> protection for the xa_state maintained in
> offset_iterate_dir(). However, there is no guarantee that will
> continue to be the case in the future.
> 
> Since offset_iterate_dir() doesn't build xa_state locally any more,
> there's no longer a strong need for offset_find_next(). Clean up by
> rolling these two helpers together.
> 
> Suggested-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
> Message-ID: <170785993027.11135.8830043889278631735.stgit@91.116.238.104.host.secureserver.net>
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/libfs.c |   12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index eec6031b0155..752e24c669d9 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -402,12 +402,13 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
>  	return vfs_setpos(file, offset, U32_MAX);
>  }
>  
> -static struct dentry *offset_find_next(struct xa_state *xas)
> +static struct dentry *offset_find_next(struct offset_ctx *octx, loff_t offset)
>  {
>  	struct dentry *child, *found = NULL;
> +	XA_STATE(xas, &octx->xa, offset);
>  
>  	rcu_read_lock();
> -	child = xas_next_entry(xas, U32_MAX);
> +	child = xas_next_entry(&xas, U32_MAX);
>  	if (!child)
>  		goto out;
>  	spin_lock(&child->d_lock);
> @@ -430,12 +431,11 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
>  
>  static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
>  {
> -	struct offset_ctx *so_ctx = inode->i_op->get_offset_ctx(inode);
> -	XA_STATE(xas, &so_ctx->xa, ctx->pos);
> +	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
>  	struct dentry *dentry;
>  
>  	while (true) {
> -		dentry = offset_find_next(&xas);
> +		dentry = offset_find_next(octx, ctx->pos);
>  		if (!dentry)
>  			return ERR_PTR(-ENOENT);
>  
> @@ -444,8 +444,8 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
>  			break;
>  		}
>  
> +		ctx->pos = dentry2offset(dentry) + 1;
>  		dput(dentry);
> -		ctx->pos = xas.xa_index + 1;
>  	}
>  	return NULL;
>  }
> 
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR