Re: [PATCH 1/2] ring-buffer: Fix event length with forced 8-byte alignment

[Hui Wang <hui.wang@canonical.com>]

On 6/9/26 00:52, Steven Rostedt wrote:
> On Mon, 8 Jun 2026 18:02:45 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
>
>> On Sun,  7 Jun 2026 15:24:30 +0800
>> Hui Wang <hui.wang@canonical.com> 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) <mhiramat@kernel.org>
> 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/20260601023251.1916483-1-dtcccc@linux.alibaba.com/
>
>
> -- 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.