Re: [PATCH] Fix blktrace unaligned memory access

[Jens Axboe]

On Fri, Jul 10 2009, Li Zefan wrote:
> Jens Axboe wrote:
> > On Fri, Jul 10 2009, Li Zefan wrote:
> >> Jens Axboe wrote:
> >>> Hi,
> >>>
> >>> It seems that relay_reserve() (or the ring_buffer_event_data(), that one
> >>> still needs some love) can return unaligned memory, there's no way
> >>> around that when you have pdu lengths that aren't a nice size. This can
> >>> cause unaligned access warnings on platforms that care about alignment.
> >>>
> >> Seems relay_reserve() does nothing for alignment..On the other hand,
> >> ring_buffer_event_data() returns a ptr which is 32bit-aligned, but
> >> this still means it can cause unaligned accesses on 64bits arch, while
> >> I think it's fixable in ring buffer, it's certainly not an easy job.
> > 
> > Right, it's a bit nasty...
> > 
> 
> Lai Jiangshan noticed this issue long ago and had some ideas in mind
> how to fix ring buffer, but never try it out for it will probably be
> frustrating..
> 
> >>> This is an RFC, perhaps we can fix this in some other way. This one takes
> >>> the simple approach, use an on-stack copy and memcpy() that to the
> >>> destination.
> >>>
> >> or get_unaligned() ?
> > 
> > put_unaligned(), you mean? The big question is then which is faster, using
> > put_unaligned() or doing the memcpy() of the structure...
> > 
> 
> Ah, I meant put_unaligned().
> 
> I think the patch you posted can be a workaround at least for now, and
> can be improved by detecting HAVE_EFFICIENT_UNALIGNED_ACCESS

We either do one or the other, I don't want to clutter the code with
both methods implemented and some ifdef checking and deciding. I'll
check the unaligned put here on x86/ppc/sparc and then decide which one
is faster.

-- 
Jens Axboe

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