relay: make create_buf_file and remove_buf_file callbacks mandatory

All clients provide create_buf_file and remove_buf_file callbacks, and
they're required for relay to make sense.  There is no point in them being
optional.

Also document whether each callback is mandatory/optional.

Link: https://lkml.kernel.org/r/88003c1527386b93036e286e7917f1e33aec84ac.1606153547.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/relay.h

@@ -89,6 +89,8 @@ struct rchan_callbacks
 	 * The client should return 1 to continue logging, 0 to stop
 	 * logging.
 	 *
+	 * This callback is optional.
+	 *
 	 * NOTE: subbuf_start will also be invoked when the buffer is
 	 *       created, so that the first sub-buffer can be initialized
 	 *       if necessary.  In this case, prev_subbuf will be NULL.
@@ -122,6 +124,8 @@ struct rchan_callbacks
 	 * cause relay_open() to create a single global buffer rather
 	 * than the default set of per-cpu buffers.
 	 *
+	 * This callback is mandatory.
+	 *
 	 * See Documentation/filesystems/relay.rst for more info.
 	 */
 	struct dentry *(*create_buf_file)(const char *filename,
@@ -139,6 +143,8 @@ struct rchan_callbacks
 	 * channel buffer.
 	 *
 	 * The callback should return 0 if successful, negative if not.
+	 *
+	 * This callback is mandatory.
 	 */
 	int (*remove_buf_file)(struct dentry *dentry);
 };

kernel/relay.c

@@ -271,26 +271,6 @@ static int subbuf_start_default_callback (struct rchan_buf *buf,
 	return 1;
 }
 
-/*
- * create_buf_file_create() default callback.  Does nothing.
- */
-static struct dentry *create_buf_file_default_callback(const char *filename,
-						       struct dentry *parent,
-						       umode_t mode,
-						       struct rchan_buf *buf,
-						       int *is_global)
-{
-	return NULL;
-}
-
-/*
- * remove_buf_file() default callback.  Does nothing.
- */
-static int remove_buf_file_default_callback(struct dentry *dentry)
-{
-	return -EINVAL;
-}
-
 /**
  *	wakeup_readers - wake up readers waiting on a channel
  *	@work: contains the channel buffer
@@ -467,10 +447,6 @@ static void setup_callbacks(struct rchan *chan,
 {
 	if (!cb->subbuf_start)
 		cb->subbuf_start = subbuf_start_default_callback;
-	if (!cb->create_buf_file)
-		cb->create_buf_file = create_buf_file_default_callback;
-	if (!cb->remove_buf_file)
-		cb->remove_buf_file = remove_buf_file_default_callback;
 	chan->cb = cb;
 }
 
@@ -530,7 +506,7 @@ struct rchan *relay_open(const char *base_filename,
 		return NULL;
 	if (subbuf_size > UINT_MAX / n_subbufs)
 		return NULL;
-	if (!cb)
+	if (!cb || !cb->create_buf_file || !cb->remove_buf_file)
 		return NULL;
 
 	chan = kzalloc(sizeof(struct rchan), GFP_KERNEL);