linux-kernel - Re: [PATCH] ring_buffer: Ensure that buffer page data is aligned.

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Message-ID: <BANLkTinO85w4G9nFjio4HtOcrS0u-Ezz1Q@mail.gmail.com>
Date:	Thu, 16 Jun 2011 14:32:41 +0100
From:	Will Newton <will.newton@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>, fweisbec@...il.com,
	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ring_buffer: Ensure that buffer page data is aligned.

On Thu, Jun 16, 2011 at 2:17 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 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?

Yes, it's likely for all 64bit arches will be aligned correctly.

>>
>> Signed-off-by: Will Newton <will.newton@...tec.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.

I have modified that conditional in my local tree so the patch is not
a nop there. Obviously it doesn't make any sense to merge that change
yet. I thought this part of the patch was a reasonable cleanup to make
the code a bit more self documenting though.
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