Re: [PATCH 1/2] vfs: split generic splice code from i_mutex locking

[Jan Kara <jack@suse.cz>]

On Mon 08-08-11 16:45:26, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> XFS holds locks that should be nested inside the inode->i_mutex when
> generic_file_splice_write is called. This function takes the
> i_mutex, and so we get a lock inversion that triggers lockdep
> warnings and has been found to cause real deadlocks.
> 
> XFS does not need the splice code to take the i_mutex to do the page
> cache manipulation, so modify the generic splice code to use an
> actor function for the code that currently requires the i_mutex to
> be taken.  Convert generic_file_splice_write() to use this new
> interface supplying a generic actor function that performs the same
> actions as the existing code.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
  The patch looks good to me.
Reviewed-by: Jan Kara <jack@suse.cz>

								Honza
> ---
>  fs/splice.c            |   58 ++++++++++++++++++++++++++++++++++++++----------
>  include/linux/splice.h |    5 ++++
>  2 files changed, 51 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/splice.c b/fs/splice.c
> index fa2defa..a02dddc 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -967,24 +967,24 @@ ssize_t splice_from_pipe(struct pipe_inode_info *pipe, struct file *out,
>  }
>  
>  /**
> - * generic_file_splice_write - splice data from a pipe to a file
> + * splice_write_to_file - splice data from a pipe to a file
>   * @pipe:	pipe info
>   * @out:	file to write to
>   * @ppos:	position in @out
>   * @len:	number of bytes to splice
>   * @flags:	splice modifier flags
> + * @actor:	worker that does the splicing from the pipe to the file
>   *
>   * Description:
>   *    Will either move or copy pages (determined by @flags options) from
>   *    the given pipe inode to the given file.
>   *
>   */
> -ssize_t
> -generic_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
> -			  loff_t *ppos, size_t len, unsigned int flags)
> +ssize_t splice_write_to_file(struct pipe_inode_info *pipe, struct file *out,
> +			     loff_t *ppos, size_t len, unsigned int flags,
> +			     splice_write_actor actor)
>  {
>  	struct address_space *mapping = out->f_mapping;
> -	struct inode *inode = mapping->host;
>  	struct splice_desc sd = {
>  		.total_len = len,
>  		.flags = flags,
> @@ -1001,13 +1001,8 @@ generic_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
>  		if (ret <= 0)
>  			break;
>  
> -		mutex_lock_nested(&inode->i_mutex, I_MUTEX_CHILD);
> -		ret = file_remove_suid(out);
> -		if (!ret) {
> -			file_update_time(out);
> -			ret = splice_from_pipe_feed(pipe, &sd, pipe_to_file);
> -		}
> -		mutex_unlock(&inode->i_mutex);
> +		ret = actor(pipe, &sd);
> +
>  	} while (ret > 0);
>  	splice_from_pipe_end(pipe, &sd);
>  
> @@ -1032,7 +1027,46 @@ generic_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
>  
>  	return ret;
>  }
> +EXPORT_SYMBOL(splice_write_to_file);
>  
> +static ssize_t generic_file_splice_write_actor(struct pipe_inode_info *pipe,
> +					       struct splice_desc *sd)
> +{
> +	struct file *out = sd->u.file;
> +	struct inode *inode = out->f_mapping->host;
> +	ssize_t ret;
> +
> +	mutex_lock_nested(&inode->i_mutex, I_MUTEX_CHILD);
> +	ret = file_remove_suid(out);
> +	if (!ret) {
> +		file_update_time(out);
> +		ret = splice_from_pipe_feed(pipe, sd, pipe_to_file);
> +	}
> +	mutex_unlock(&inode->i_mutex);
> +
> +	return ret;
> +}
> +
> +/**
> + * generic_file_splice_write - splice data from a pipe to a file
> + * @pipe:	pipe info
> + * @out:	file to write to
> + * @ppos:	position in @out
> + * @len:	number of bytes to splice
> + * @flags:	splice modifier flags
> + *
> + * Description:
> + *    Will either move or copy pages (determined by @flags options) from
> + *    the given pipe inode to the given file.
> + *
> + */
> +ssize_t
> +generic_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
> +			  loff_t *ppos, size_t len, unsigned int flags)
> +{
> +	return splice_write_to_file(pipe, out, ppos, len, flags,
> +				    generic_file_splice_write_actor);
> +}
>  EXPORT_SYMBOL(generic_file_splice_write);
>  
>  static int write_pipe_buf(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
> diff --git a/include/linux/splice.h b/include/linux/splice.h
> index 26e5b61..bd14281 100644
> --- a/include/linux/splice.h
> +++ b/include/linux/splice.h
> @@ -61,6 +61,8 @@ typedef int (splice_actor)(struct pipe_inode_info *, struct pipe_buffer *,
>  			   struct splice_desc *);
>  typedef int (splice_direct_actor)(struct pipe_inode_info *,
>  				  struct splice_desc *);
> +typedef ssize_t (splice_write_actor)(struct pipe_inode_info *,
> +				  struct splice_desc *);
>  
>  extern ssize_t splice_from_pipe(struct pipe_inode_info *, struct file *,
>  				loff_t *, size_t, unsigned int,
> @@ -82,6 +84,9 @@ extern ssize_t splice_to_pipe(struct pipe_inode_info *,
>  extern ssize_t splice_direct_to_actor(struct file *, struct splice_desc *,
>  				      splice_direct_actor *);
>  
> +extern ssize_t splice_write_to_file(struct pipe_inode_info *, struct file *,
> +				    loff_t *, size_t, unsigned int,
> +				    splice_write_actor *);
>  /*
>   * for dynamic pipe sizing
>   */
> -- 
> 1.7.5.4
> 
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-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR