Messages in this thread | | | Date | Fri, 10 Jul 2009 17:30:28 +0800 | From | Li Zefan <> | Subject | Re: [PATCH] Fix blktrace unaligned memory access |
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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
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