linux-kernel - Re: [PATCH][GIT PULL][for 2.6.35] tracing: Add alignment to syscall metadata declarations

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Message-ID: <20100709220252.GA14080@merkur.ravnborg.org>
Date:	Sat, 10 Jul 2010 00:02:52 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Zeev Tarantov <zeev.tarantov@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Maciej@...ispam.struernethosting.dk
Subject: Re: [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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