On 6/9/26 00:52, Steven Rostedt wrote:
On Mon, 8 Jun 2026 18:02:45 +0900
Masami Hiramatsu (Google) <[email protected]> wrote:
On Sun, 7 Jun 2026 15:24:30 +0800
Hui Wang <[email protected]> wrote:
When RB_FORCE_8BYTE_ALIGNMENT is true, rb_calculate_event_length()
reserves the space of event->array[0] for placing the data length and
rb_update_event() stores the data length in event->array[0]
accordingly. As a result the whole event length will add extra 4 bytes
for sizeof(event.array[0]) unconditionally.
But ring_buffer_event_length() only subtracts the
sizeof(event->array[0]) for events larger than RB_MAX_SMALL_DATA +
sizeof(event->array[0]). As a result, small events on architectures
with RB_FORCE_8BYTE_ALIGNMENT=true report a data length that is 4
bytes larger than expected.
To fix it, add the RB_FORCE_8BYTE_ALIGNMENT as a condition to subtract
the size of that length field whenever RB_FORCE_8BYTE_ALIGNMENT is
true.
This issue is observed in a riscv64 kernel with
CONFIG_HAVE_64BIT_ALIGNED_ACCESS set to y, when we run ftrace selftest
trace_marker_raw.tc, we get the weird log: for cases where the id is
1..100, the number of data field is 8*N, but once id exceeds 100, the
number of data field becomes 8*N+4:
# 1 buf: 58 00 00 00 80 5e d1 63 (number of data field is 8*1)
...
# a buf: 58 ... (number of data field is 8*2)
...
# 64 buf: 58 ... (number of data field is 8*13)
# 65 buf: 58 ... (number of data field is 8*13+4)
After applying this change, the number of data field keeps being 8*N+4
consistently.
Good catch!
This looks good to me.
Reviewed-by: Masami Hiramatsu (Google) <[email protected]>
This is the patch I meant to reply to.
NACK as the test is broken and not the kernel.
There's a pending fix already:
https://lore.kernel.org/all/[email protected]/
-- Steve
Hi Steven,
Thanks for the pointer. I reverted my two patches and applied the patch
you referenced, but unfortunately it doesn't resolve the problem — the
testcase still fails in my environment (riscv64 kernel with
CONFIG_HAVE_64BIT_ALIGNED_ACCESS enabled).
From what I can tell, that fix addresses a different problem than the
one I'm hitting: it targets a 64K page-size issue, whereas my failure is
caused by the 64-bit alignment requirement
(CONFIG_HAVE_64BIT_ALIGNED_ACCESS). So I don't think they're the same
root cause.
So can you please take a look at them again.
Thanks,
Hui.
