This is a note to let you know that I've just added the patch titled
ring-buffer: Test last update in 32bit version of __rb_time_read()
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-test-last-update-in-32bit-version-of-__rb_time_read.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 f458a1453424e03462b5bb539673c9a3cddda480 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
Date: Wed, 6 Dec 2023 10:00:50 -0500
Subject: ring-buffer: Test last update in 32bit version of __rb_time_read()
From: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
commit f458a1453424e03462b5bb539673c9a3cddda480 upstream.
Since 64 bit cmpxchg() is very expensive on 32bit architectures, the
timestamp used by the ring buffer does some interesting tricks to be able
to still have an atomic 64 bit number. It originally just used 60 bits and
broke it up into two 32 bit words where the extra 2 bits were used for
synchronization. But this was not enough for all use cases, and all 64
bits were required.
The 32bit version of the ring buffer timestamp was then broken up into 3
32bit words using the same counter trick. But one update was not done. The
check to see if the read operation was done without interruption only
checked the first two words and not last one (like it had before this
update). Fix it by making sure all three updates happen without
interruption by comparing the initial counter with the last updated
counter.
Link: https://lore.kernel.org/linux-trace-kernel/20231206100050.3100b7bb@xxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Fixes: f03f2abce4f39 ("ring-buffer: Have 32 bit time stamps use all 64 bits")
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/trace/ring_buffer.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -646,8 +646,8 @@ static inline bool __rb_time_read(rb_tim
*cnt = rb_time_cnt(top);
- /* If top and bottom counts don't match, this interrupted a write */
- if (*cnt != rb_time_cnt(bottom))
+ /* If top and msb counts don't match, this interrupted a write */
+ if (*cnt != rb_time_cnt(msb))
return false;
/* The shift to msb will lose its cnt bits */
Patches currently in stable-queue which might be from rostedt@xxxxxxxxxxx are
queue-6.1/tracing-fix-a-possible-race-when-disabling-buffered-events.patch
queue-6.1/tracing-fix-incomplete-locking-when-disabling-buffered-events.patch
queue-6.1/ring-buffer-test-last-update-in-32bit-version-of-__rb_time_read.patch
queue-6.1/tracing-fix-a-warning-when-allocating-buffered-event.patch
queue-6.1/tracing-disable-snapshot-buffer-when-stopping-instance-tracers.patch
queue-6.1/tracing-always-update-snapshot-buffer-size.patch
