ring-buffer: Correct stale comments related to non-consuming readers

Adjust the following code documentation:

* Kernel-doc comments for ring_buffer_read_prepare() and
  ring_buffer_read_finish() mention that recording to the ring buffer is
  disabled when the read is active. Remove mention of this restriction
  because it was already lifted in commit 1039221 ("ring-buffer: Do
  not disable recording when there is an iterator").

* Function ring_buffer_read_finish() performs a self-check of the
  ring-buffer by locking cpu_buffer->reader_lock and then calling
  rb_check_pages(). The preceding comment explains that the lock is
  needed because rb_check_pages() clears the HEAD flag required by
  readers which might be running in parallel. Remove this explanation
  because commit 8843e06 ("ring-buffer: Handle race between
  rb_move_tail and rb_check_pages") simplified the function so it no
  longer resets the mentioned flag. Nonetheless, the lock is still
  needed because a reader swapping a page into the ring buffer can make
  the underlying doubly-linked list temporarily inconsistent.

This is a non-functional change.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: Masami Hiramatsu <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Signed-off-by: Petr Pavlu <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: petrpavlu

kernel/trace/ring_buffer.c

@@ -5046,13 +5046,9 @@ EXPORT_SYMBOL_GPL(ring_buffer_consume);
  * @flags: gfp flags to use for memory allocation
  *
  * This performs the initial preparations necessary to iterate
- * through the buffer.  Memory is allocated, buffer recording
+ * through the buffer.  Memory is allocated, buffer resizing
  * is disabled, and the iterator pointer is returned to the caller.
  *
- * Disabling buffer recording prevents the reading from being
- * corrupted. This is not a consuming read, so a producer is not
- * expected.
- *
  * After a sequence of ring_buffer_read_prepare calls, the user is
  * expected to make at least one call to ring_buffer_read_prepare_sync.
  * Afterwards, ring_buffer_read_start is invoked to get things going
@@ -5139,21 +5135,15 @@ EXPORT_SYMBOL_GPL(ring_buffer_read_start);
  * ring_buffer_read_finish - finish reading the iterator of the buffer
  * @iter: The iterator retrieved by ring_buffer_start
  *
- * This re-enables the recording to the buffer, and frees the
- * iterator.
+ * This re-enables resizing of the buffer, and frees the iterator.
  */
 void
 ring_buffer_read_finish(struct ring_buffer_iter *iter)
 {
 	struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
 	unsigned long flags;
 
-	/*
-	 * Ring buffer is disabled from recording, here's a good place
-	 * to check the integrity of the ring buffer.
-	 * Must prevent readers from trying to read, as the check
-	 * clears the HEAD page and readers require it.
-	 */
+	/* Use this opportunity to check the integrity of the ring buffer. */
 	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
 	rb_check_pages(cpu_buffer);
 	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);