Subject: Re: [PATCH 03/15] x86: remove early_gdt_descr reference
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Glauber Costa <gcosta@redhat.com>
Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
       tglx@linutronix.de, mingo@elte.hu, hugh@veritas.com
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Date: Mon, 09 Jun 2008 12:40:02 -0500
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On Mon, 2008-06-09 at 14:23 -0300, Glauber Costa wrote:
> James Bottomley wrote:
> > On Mon, 2008-06-09 at 12:49 -0300, Glauber Costa wrote:
> >> James Bottomley wrote:
> >>> On Mon, 2008-06-09 at 11:16 -0300, Glauber Costa wrote:
> >>>> since we use switch_to_new_gdt, there is no point
> >>>> in assigning early_gdt_descr except for the first
> >>>> assignment, which is done manually.
> >>> What makes you think you can do this?  If you don't update the early
> >>> boot gdt, they all end up using the Boot CPU one.  The problem with this
> >>> is that there's a time from start_secondary to switch_to_new_gdt where
> >>> the per cpu selector (%fs) and the pda selector (%gs) are those of the
> >>> boot CPU.   The former isn't a problem but the CPU number is in the
> >>> latter, and it's used in that path before we get to the initialisation.
> >> You are right, I missed it.
> >>
> >> However, it only seem to be used in cpu_init, and very early. Sure there 
> >> are some users _before_ we load the new gdt, but nothing prevents them 
> >> to be moved after it. (Of course, this patch is wrong anyway).
> >>
> >> And if we do that, we can even take the %fs loading out of head_32.S
> >> Of course, it's only valid if those are indeed the only early users of it.
> >>
> >> Is there any other use I'm missing?
> > 
> > Well, %fs loading there is done for the boot CPU. To eliminate that you
> > have to not only verify that start_secondary doesn't use anything in
> > per_cpu areas, but also verify that nothing in start_kernel() up until
> > boot_cpu_init() does ... That's a lot of smp_processor_id() references
> > to convert.
> Yes, after a second look, it would be tricky indeed. But only for cpu0.
> For all the others, I still think we could get rid of the problem by 
> switching to the new gdt earlier in cpu_init.
> 
> What do you think?

Operating a CPU with a bogus GDT is very fragile.  You can fix all the
current issues with the secondary CPUs, but it gives a critical section
within which none of the per_cpu operations will work.  It only takes
one patch violating this rule and we have a very subtle bug introduced.

It looks to me like the better fix might be just to initialise the gdt
completely and properly in do_boot_cpu and just have the single switch
in head_32.S be the correct one.  That way there's no problem critical
region.

James


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