linux-kernel - Re: [PATCH 04/15] ftrace: pack event structures.

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Message-ID: <1299683884.15854.154.camel@gandalf.stny.rr.com>
Date:	Wed, 09 Mar 2011 10:18:04 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	David Miller <davem@...emloft.net>
Cc:	linux-kernel@...r.kernel.org, mrubin@...gle.com, dhsharp@...gle.com
Subject: Re: [PATCH 04/15] ftrace: pack event structures.

On Tue, 2011-03-08 at 22:39 -0800, David Miller wrote:
> From: Steven Rostedt <rostedt@...dmis.org>
> Date: Tue, 08 Mar 2011 18:30:03 -0500
> 
> > I'm going back through this patch set and looking to incorporate it into
> > 2.6.39. But I know how touchy sparc is with "packed" data structures. Do
> > you see this patch as hurting sparc?
> > 
> > I wonder if we should change this to something like:
> > 
> > #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> > # define do_event_packed __attirbute__((packed))
> > #else
> > # define do_event_packed
> > #endif
> > 
> > and use "do_event_packed" instead?
> 
> I think you should elide packed at all costs, and instead tell the compiler
> what your intentions are by exposing the real types using unions or
> similar instead of hiding it in an opaque way behind final char[] arrays.

The problem here is not the issue of char[] but because of the way
ftrace's header was 12 bytes and caused everything that had a 8byte word
use another 4 bytes to align it. As I stated in another email, I think
removing the "int lock_depth" from the ftrace header should solve this.

David (Sharp), the reason for the "packed" was because of that extra 4
bytes in the ftrace header, right? So removing the lock_depth should fix
the issues that you've seen?

-- Steve

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