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SubjectRe: [PATCH][GIT PULL][for 2.6.35] tracing: Add alignment to syscall metadata declarations
On Fri, Jul 09, 2010 at 05:05:50PM -0400, Steven Rostedt wrote:
> On Fri, 2010-07-09 at 22:53 +0200, Sam Ravnborg wrote:
>
> > But in this case we have no control of the value of "." (current address)
> > when we have processed (_ftrace_events) so it may even be at a 2 byte boundary.
> > The linker will add padding as needed to satisfy the alignmnet of
> > __syscalls_metadata - but that padding will be inbetween "." and the first
> > member in __syscalls_metadata.
>
> Fine, but this is a separate issue. I doubt the "ALIGN(8);" would have
> helped us anyway. Remember what the issue we had:
>
> ffffffff8173c438 <__start_syscalls_metadata>:
> ...
>
> ffffffff8173c440 <__syscall_meta__mmap>:
>
>
> __start_syscalls_metadata was already aligned to 8, but for some strange
> reason, gcc decided to align the first member to 16.

I found some more info in following bug:
https://bugzillafiles.novell.org/attachment.cgi?id=344563

Jeff Mahoney says:
GCC 4.5 introduced behavior that forces the alignment of structures to
use the largest possible value. The default value is 32 bytes, so if
some structures are defined with a 4-byte alignment and others aren't
declared with an alignment constraint at all - it will align at 32-bytes.

So according to this we can rely on 32 byte alignment and
this is trivial to add in vmlinux.lds.h.

Sam


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