linux-kernel - Re: [PATCH RFC 0/6] Implement per-processor data areas for i386.

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Message-ID: <200608300838_MC3-1-C9C6-CA79@compuserve.com>
Date:	Wed, 30 Aug 2006 08:33:40 -0400
From:	Chuck Ebbert <76306.1226@...puserve.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Zachary Amsden <zach@...are.com>,
	Jan Beulich <jbeulich@...ell.com>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH RFC 0/6] Implement per-processor data areas for
  i386.

In-Reply-To: <44F557A8.1030605@...p.org>

On Wed, 30 Aug 2006 02:17:28 -0700, Jeremy Fitzhardinge wrote:

> > This changes the ABI for signals and ptrace() and that seems like
> > a bad idea to me.
> >   
> 
> I don't believe it does; it certainly shouldn't change the usermode 
> ABI.  How do you see it changing?

Nevermind.  I thought because you changed struct pt_regs in ptrace_abi.h
it meant a user ABI change.

> > And the way things are done now is so ingrained into the i386
> > kernel that I'm not sure it can be done.  E.g. I found two
> > open-coded implementations of current, one in kernel_fpu_begin()
> > and one in math_state_restore().
> >   
> 
> That's OK.  The current task will still be available in thread_info; 

But they can get out of sync, e.g. when switch_to() restores the new
task's esp, the PDA still contains the old pcurrent and they don't get
synchronized until the write_pda() in __switch_to().

> To be honest, I haven't looked at percpu.h in great detail.  I was 
> making assumptions about how it works, but it looks like they were wrong.

Would it make any sense to replace the 'cpu' field in thread_info with
a pointer to a PDA-like structure?  We could even embed the static per_cpu
data directly into that struct instead of chasing pointers...

-- 
Chuck

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