From ea70a9628e93168bb5a07b35f7b9a162a65afe9e Mon Sep 17 00:00:00 2001 From: Petr Pavlu Date: Fri, 17 May 2024 15:40:07 +0200 Subject: [PATCH] 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 1039221cc278 ("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 8843e06f67b1 ("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/20240517134008.24529-2-petr.pavlu@suse.com Cc: Masami Hiramatsu Cc: Mathieu Desnoyers Signed-off-by: Petr Pavlu Signed-off-by: Steven Rostedt (Google) --- kernel/trace/ring_buffer.c | 16 +++------------- 1 file changed, 3 insertions(+), 13 deletions(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 7345a8b625fb..42227727a49d 100644 --- a/kernel/trace/ring_buffer.c +++ b/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 @@ -5139,8 +5135,7 @@ 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) @@ -5148,12 +5143,7 @@ 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); -- 2.39.5