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    SubjectRe: Bug#609371: linux-image-2.6.37-trunk-sparc64: module scsi_mod: Unknown relocation: 36
    On Mon, Jan 17, 2011 at 07:07:55AM +0100, David Miller wrote:
    > From: David Miller <davem@davemloft.net>
    > 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@axis.com>
    > 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@suse.com>
    > 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@axis.com


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