Re: [PATCH] Fix blktrace unaligned memory access

[Jens Axboe <jens.axboe@oracle.com>]

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...

> > 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...

-- 
Jens Axboe