splice: remove i_mutex locking in splice_from_pipe()

splice_from_pipe() is only called from two places:

  - generic_splice_sendpage()
  - splice_write_null()

Neither of these require i_mutex to be taken on the destination inode.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

fs/splice.c

@@ -784,7 +784,7 @@ EXPORT_SYMBOL(__splice_from_pipe);
  * @actor:	handler that splices the data
  *
  * Description:
- *    See __splice_from_pipe. This function locks the input and output inodes,
+ *    See __splice_from_pipe. This function locks the pipe inode,
  *    otherwise it's identical to __splice_from_pipe().
  *
  */
@@ -793,7 +793,6 @@ ssize_t splice_from_pipe(struct pipe_inode_info *pipe, struct file *out,
 			 splice_actor *actor)
 {
 	ssize_t ret;
-	struct inode *inode = out->f_mapping->host;
 	struct splice_desc sd = {
 		.total_len = len,
 		.flags = flags,
@@ -801,24 +800,11 @@ ssize_t splice_from_pipe(struct pipe_inode_info *pipe, struct file *out,
 		.u.file = out,
 	};
 
-	/*
-	 * The actor worker might be calling ->write_begin and
-	 * ->write_end. Most of the time, these expect i_mutex to
-	 * be held. Since this may result in an ABBA deadlock with
-	 * pipe->inode, we have to order lock acquiry here.
-	 *
-	 * Outer lock must be inode->i_mutex, as pipe_wait() will
-	 * release and reacquire pipe->inode->i_mutex, AND inode must
-	 * never be a pipe.
-	 */
-	WARN_ON(S_ISFIFO(inode->i_mode));
-	mutex_lock_nested(&inode->i_mutex, I_MUTEX_PARENT);
 	if (pipe->inode)
-		mutex_lock_nested(&pipe->inode->i_mutex, I_MUTEX_CHILD);
+		mutex_lock(&pipe->inode->i_mutex);
 	ret = __splice_from_pipe(pipe, &sd, actor);
 	if (pipe->inode)
 		mutex_unlock(&pipe->inode->i_mutex);
-	mutex_unlock(&inode->i_mutex);
 
 	return ret;
 }