On Mon, Jan 17, 2011 at 07:07:55AM +0100, David Miller wrote:
> From: David Miller <davem@xxxxxxxxxxxxx>
> Date: Sat, 15 Jan 2011 21:17:22 -0800 (PST)
>
> [ Please, everyone, retain the full CC: on all replies, thanks. Some
> people are replying only into the debian bug alias, and that loses
> information and exposure for fixing this bug. ]
>
> > I think the problem we have here is that the _ftrace_events section is
> > not aligned sufficiently. That ".align 4" mnemonic is a good indication
> > of this. It should at least "8" on sparc64.
>
> I did some more research.
>
> Although I've seen commentary to the contrary, in fact using a too-small
> __attribute__((aligned())) directive will lower the alignment of data
> members, and yes that means it will lower the alignemnt to be below the
> natural and required alignment for the given type.
>
> So if you have, on 64-bit:
>
> struct foo {
> void *bar;
> };
>
> static struct foo test __attribute__((__aligned__(4)));
>
> The compiler will emit "test" with 4-byte alignment into the data
> section, even though 8-byte alignment is required for "test.bar"
>
> Assuming we wanted that to actually happen, the GCC manual is very
> explicit to state that in order for this to work, such down-aligned
> data structures must also use the "packed" attribute.
>
> I think we want none of this, and I think we should elide the align
> directives entirely, or at least fix them so we don't get unaligned
> stuff on 64-bit.
>
> Ugh, and I just noticed that include/linux/klist.h does this fixed
> alignment of "4" too, where is this stuff coming from? It's
> wrong on 64-bit, at best. But I can't see the impetus behind doing
> this at all in the first place.
>
> Oh, this is some CRIS thing, because it only byte aligns. See:
>
> commit c0e69a5bbc6fc74184aa043aadb9a53bc58f953b
> Author: Jesper Nilsson <Jesper.Nilsson@xxxxxxxx>
> Date: Wed Jan 14 11:19:08 2009 +0100
>
> klist.c: bit 0 in pointer can't be used as flag
>
> That's where the klist one comes from.
Yup, this one could instead be solved by introducing a "flags" field
in the struct, but that was considered a too large impact fix.
> The ftrace ones come from:
>
> commit 86c38a31aa7f2dd6e74a262710bf8ebf7455acc5
> Author: Jeff Mahoney <jeffm@xxxxxxxx>
> Date: Wed Feb 24 13:59:23 2010 -0500
>
> tracing: Fix ftrace_event_call alignment for use with gcc 4.5
>
> We really can't handle this that way, it's going to break stuff
> on 64-bit systems at the very least.
>
> How about we use __BIGGEST_ALIGNMENT__ or something arch-defined value
> instead?
>From CRIS-standpoint that would be fine.
/^JN - Jesper Nilsson
--
Jesper Nilsson -- jesper.nilsson@xxxxxxxx
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