[PATCH] ARM: mm: ensure TTBR0 is restored when changing ASID on rollover

[Russell King - ARM Linux]

On Wed, Jun 08, 2011 at 10:12:23PM +0100, Catalin Marinas wrote:
> On 8 June 2011 21:36, Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:
> > On Wed, Jun 08, 2011 at 09:23:23PM +0100, Will Deacon wrote:
> >> On Wed, Jun 08, 2011 at 09:01:06PM +0100, Russell King - ARM Linux wrote:
> >> > However, these patches are introducing a brand new race between the
> >> > switch_mm code and the reset_context code.
> >> >
> >> > With the new switch_mm() code, we switch TTBR0 to be the same as TTBR1.
> >> > If we then receive an IPI for reset_context(), we will change TTBR0
> >> > to point at a set of page tables which don't contain just global mappings.
> >> >
> >> > After returning from reset_context(), we will resume switch_mm(), and
> >> > change the ASID value with the page tables pointing to non-global
> >> > mappings, violating the whole reason for the switch_mm() change.
> >>
> >> Whilst this is a new race condition, it is analagous to the one we have
> >> already and could be fixed at the same time.
> >
> > Ok, I think we should revert the original patches then.  They were rushed
> > in during the merge window, and as can be seen, rushing in patches because
> > we _think_ they're right is never the correct thing to do - we've ended
> > up with a completely broken situation as stuff now stands.
> 
> We rushed a series of patches fixing this but you didn't like the
> patch disabling interrupts around cpu_switch_mm().

No.  We rushed the entire thing as can be seen due to the "forgotten"
patch and the submission of it _during_ the merge window.  Clearly
no one had thought enough about the inter-relationship between the
patches when deciding that only some of the patches should go in.

That's why there's a big reason to avoid merging any new patches during
the merge window - it gives time for such things to be found before it
becomes a problem.

> Please note that the old switch_mm code with reserved ASID is broken
> on A15 (and not just in theory), hence the need to use reserved TTBR0.

But A15 is not actively supported in mainline yet so that's not a
decision relevant to what we do with mainline.

> > Let's take out these changes and sort it out properly - not only do we
> > need to sort out these problems but we should also get rid of the
> > __ARCH_WANT_INTERRUPTS_ON_CTXSW thing completely.  I have a patch which
> > I've only tested on SA-1110 which does this so far, but it needs a little
> > more work to clean up some stuff.
> 
> Even if you get rid of __ARCH_WANT_INTERRUPTS_ON_CTXSW, I would much
> prefer to use the new switch_mm code as a base rather than going back
> to the reserved ASID. The simplest way to fix the race condition you
> mentioned is to also integrate the other patch from Will which
> disables the interrupts around cpu_switch_mm(). After that we have
> more time to review the __ARCH_WANT_INTERRUPTS_ON_CTXSW patch.

Once we have the context switching situation sorted for VIVT we can then
reconsider these patches - their pre-requisits will have been solved by
way of the VIVT situation being sorted.