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SubjectRe: [PATCH] ring_buffer: Ensure that buffer page data is aligned.
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On Thu, 2011-06-16 at 14:03 +0100, Will Newton wrote:
> Explicitly align the start of the buffer page data array to the
> required arch alignment. This is required for architectures that
> require 8 byte alignment but do not have a 8 byte local_t.

What arch does that? A 32bit arch that forces 8 byte alignment?

>
> Signed-off-by: Will Newton <will.newton@imgtec.com>
> ---
> kernel/trace/ring_buffer.c | 4 +++-
> 1 files changed, 3 insertions(+), 1 deletions(-)
>
> I don't believe that any currently in-tree architecture is affected by
> this, but it can be an issue on 32bit architectures that require an
> 8 byte aligment but only have a 32bit local_t. I think it's potentially
> cleaner to make the alignment explicit anyway.
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 0ef7b4b..36d5699 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -215,6 +215,8 @@ EXPORT_SYMBOL_GPL(tracing_is_on);
> # define RB_ARCH_ALIGNMENT 8U
> #endif
>
> +#define RB_ALIGN_DATA __aligned(RB_ARCH_ALIGNMENT)
> +

Note, the code above this is:

#define RB_EVNT_HDR_SIZE (offsetof(struct ring_buffer_event, array))
#define RB_ALIGNMENT 4U
#define RB_MAX_SMALL_DATA (RB_ALIGNMENT * RINGBUF_TYPE_DATA_TYPE_LEN_MAX)
#define RB_EVNT_MIN_SIZE 8U /* two 32bit words */

#if !defined(CONFIG_64BIT) || defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
# define RB_FORCE_8BYTE_ALIGNMENT 0
# define RB_ARCH_ALIGNMENT RB_ALIGNMENT
#else
# define RB_FORCE_8BYTE_ALIGNMENT 1
# define RB_ARCH_ALIGNMENT 8U
#endif


Which means that when CONFIG_64BIT is not set, RB_ARCH_ALIGNMENT is 4.
This means that this patch is really a nop and doesn't do anything for
your arch.

-- Steve

> /* define RINGBUF_TYPE_DATA for 'case RINGBUF_TYPE_DATA:' */
> #define RINGBUF_TYPE_DATA 0 ... RINGBUF_TYPE_DATA_TYPE_LEN_MAX
>
> @@ -363,7 +365,7 @@ EXPORT_SYMBOL_GPL(ring_buffer_event_data);
> struct buffer_data_page {
> u64 time_stamp; /* page time stamp */
> local_t commit; /* write committed index */
> - unsigned char data[]; /* data of buffer page */
> + unsigned char data[] RB_ALIGN_DATA; /* data of buffer page */
> };
>
> /*




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