[PATCH v2] ring-buffer: Never use absolute timestamp for first event

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>

On 32bit machines, the 64 bit timestamps are broken up into 32 bit words
to keep from using local64_cmpxchg(), as that is very expensive on 32 bit
architectures.

On 32 bit architectures, reading these timestamps can happen in a middle
of an update. In this case, the read returns "false", telling the caller
that the timestamp is in the middle of an update, and it needs to assume
it is corrupted. The code then accommodates this.

When first reserving space on the ring buffer, a "before_stamp" and
"write_stamp" are read. If they do not match, or if either is in the
process of being updated (false was returned from the read), an absolute
timestamp is added and the delta is not used, as that requires reading
theses timestamps without being corrupted.

The one case that this does not matter is if the event is the first event
on the sub-buffer, in which case, the event uses the sub-buffer's
timestamp and doesn't need the other stamps for calculating them.

After some work to consolidate the code, if the before or write stamps are
in the process of updating, an absolute timestamp will be added regardless
if the event is the first event on the sub-buffer. This is wrong as it
should not care about the success of these reads if it is the first event
on the sub-buffer.

Fix up the parenthesis so that even if the timestamps are corrupted, if
the event is the first event on the sub-buffer (w == 0) it still does not
force an absolute timestamp.

It's actually likely that w is not zero, but move it out of the unlikeyl()
and test it first. It should be in hot cache anyway, and there's no reason
to do the rest of the test for the first event on the sub-buffer. And this
prevents having to test all the 'or' statements in that case.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 58fbc3c63275c ("ring-buffer: Consolidate add_timestamp to remove some branches")
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
Changes since v2: https://lore.kernel.org/linux-trace-kernel/20231211115949.4692e429@xxxxxxxxxxxxxxxxxx

- Move the test to 'w' out of the unlikely and do it first.
  It's already in hot cache, and the rest of test shouldn't be done
  if 'w' is zero.

 kernel/trace/ring_buffer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index b416bdf6c44a..095b86081ea8 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3581,7 +3581,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
 		 * absolute timestamp.
 		 * Don't bother if this is the start of a new page (w == 0).
 		 */
-		if (unlikely(!a_ok || !b_ok || (info->before != info->after && w))) {
+		if (w && unlikely(!a_ok || !b_ok || info->before != info->after)) {
 			info->add_timestamp |= RB_ADD_STAMP_FORCE | RB_ADD_STAMP_EXTEND;
 			info->length += RB_LEN_TIME_EXTEND;
 		} else {
-- 
2.42.0