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SubjectRe: [RFC][PATCH] linux-2.6.0-test2_mtrr-race-fix_A0
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On Wed, 2003-08-06 at 10:15, john stultz wrote:
> On Wed, 2003-08-06 at 01:52, Mathias Fröhlich wrote:
> > Hi,
> >
> > You should not remove the barrier past mtrr change. If you do that, it is
> > possible that cpu's run with inconsistent mtrrs. This can have bad
> > sideeffects since at least the cache snooping protocol used by intel uses
> > assumptions about the cachability of memory regions. Those information about
> > the cachability is also taken from the mtrrs as far as I remember.
> > This intel cpu developer manual, which documented the early PII and PPro
> > chips, recommended this algorithm. Since actual intel cpus use the same old
> > cpu to chipset bus protocol, this old documentation most propably still
> > applies.
>
> Hmm. I should dig up that doc. Its a little hazy in my mind, but I think
> I understand your description. I'm glad you caught this, as I can't
> imagine the subtle bugs that might have popped up.
>
> Well, let me look at it again and see if I can come up with a proper
> fix.
>
> Thanks for the knowledgeable feedback and sanity checking!
> -john


I added an extra sync up from the caller after the last gate change so
it is the last one to touch the automatic data.

Mark.


===== arch/i386/kernel/cpu/mtrr/main.c 1.29 vs edited =====
--- 1.29/arch/i386/kernel/cpu/mtrr/main.c Tue Jul 15 10:08:48 2003
+++ edited/arch/i386/kernel/cpu/mtrr/main.c Wed Aug 6 10:46:00 2003
@@ -169,6 +169,7 @@
cpu_relax();
barrier();
}
+ atomic_dec(&data->count);
local_irq_restore(flags);
}

@@ -256,8 +257,18 @@
cpu_relax();
barrier();
}
- local_irq_restore(flags);
+ atomic_set(&data.count, num_booting_cpus() - 1);
atomic_set(&data.gate,0);
+
+ /*
+ * Wait here for everyone to have seen the gate change
+ * So we're the last ones to touch 'data'
+ */
+ while(atomic_read(&data.count)) {
+ cpu_relax();
+ barrier();
+ }
+ local_irq_restore(flags);
}

/**

--
Mark Haverkamp <markh@osdl.org>

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