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

On Wed, Jun 08, 2011 at 09:36:15PM +0100, Russell King - ARM Linux wrote:

On Wed, Jun 08, 2011 at 09:23:23PM +0100, Will Deacon wrote:

quoted

On Wed, Jun 08, 2011 at 09:01:06PM +0100, Russell King - ARM Linux wrote:

quoted

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.

Seems a shame given that disabling interrupts during switch_mm would fix this,
but yes, reverting them is certainly better than having a broken kernel. I
think you just need to revert 6944/1 and 6943/1; the other two from that series
are fine to be left in.

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.

Could you post that code please? I can then rebase these patches against it for
future inclusion in mainline. Does the removal of
__ARCH_WANT_INTERRUPTS_ON_CTXSW mean that switch_mm will run with interrupts
disabled by default?

So, we're weren't - and still aren't - ready for any of these changes.

Once the interrupt stuff is sorted out I can rebase the ASID stuff on top of it
and we should be ready to go.

Cheers,

Will