This is a note to let you know that I've just added the patch titled
ring-buffer: Do not update before stamp when switching sub-buffers
to the 6.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
ring-buffer-do-not-update-before-stamp-when-switching-sub-buffers.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 9e45e39dc249c970d99d2681f6bcb55736fd725c Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
Date: Mon, 11 Dec 2023 11:44:20 -0500
Subject: ring-buffer: Do not update before stamp when switching sub-buffers
From: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
commit 9e45e39dc249c970d99d2681f6bcb55736fd725c upstream.
The ring buffer timestamps are synchronized by two timestamp placeholders.
One is the "before_stamp" and the other is the "write_stamp" (sometimes
referred to as the "after stamp" but only in the comments. These two
stamps are key to knowing how to handle nested events coming in with a
lockless system.
When moving across sub-buffers, the before stamp is updated but the write
stamp is not. There's an effort to put back the before stamp to something
that seems logical in case there's nested events. But as the current event
is about to cross sub-buffers, and so will any new nested event that happens,
updating the before stamp is useless, and could even introduce new race
conditions.
The first event on a sub-buffer simply uses the sub-buffer's timestamp
and keeps a "delta" of zero. The "before_stamp" and "write_stamp" are not
used in the algorithm in this case. There's no reason to try to fix the
before_stamp when this happens.
As a bonus, it removes a cmpxchg() when crossing sub-buffers!
Link: https://lore.kernel.org/linux-trace-kernel/20231211114420.36dde01b@xxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Fixes: a389d86f7fd09 ("ring-buffer: Have nested events still record running time stamp")
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/trace/ring_buffer.c | 9 +--------
1 file changed, 1 insertion(+), 8 deletions(-)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3601,14 +3601,7 @@ __rb_reserve_next(struct ring_buffer_per
/* See if we shot pass the end of this buffer page */
if (unlikely(write > BUF_PAGE_SIZE)) {
- /* before and after may now different, fix it up*/
- b_ok = rb_time_read(&cpu_buffer->before_stamp, &info->before);
- a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after);
- if (a_ok && b_ok && info->before != info->after)
- (void)rb_time_cmpxchg(&cpu_buffer->before_stamp,
- info->before, info->after);
- if (a_ok && b_ok)
- check_buffer(cpu_buffer, info, CHECK_FULL_PAGE);
+ check_buffer(cpu_buffer, info, CHECK_FULL_PAGE);
return rb_move_tail(cpu_buffer, tail, info);
}
Patches currently in stable-queue which might be from rostedt@xxxxxxxxxxx are
queue-6.1/ring-buffer-do-not-update-before-stamp-when-switching-sub-buffers.patch
queue-6.1/ring-buffer-have-saved-event-hold-the-entire-event.patch
queue-6.1/ring-buffer-have-rb_time_cmpxchg-set-the-msb-counter-too.patch
queue-6.1/ring-buffer-do-not-try-to-put-back-write_stamp.patch
queue-6.1/ring-buffer-fix-memory-leak-of-free-page.patch
queue-6.1/ring-buffer-fix-a-race-in-rb_time_cmpxchg-for-32-bit-archs.patch
queue-6.1/tracing-update-snapshot-buffer-on-resize-if-it-is-allocated.patch
queue-6.1/ring-buffer-fix-writing-to-the-buffer-with-max_data_size.patch
