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

From: linux@arm.linux.org.uk (Russell King - ARM Linux)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH] ARM: mm: ensure TTBR0 is restored when changing ASID on rollover
Date: Wed, 8 Jun 2011 21:36:15 +0100	[thread overview]
Message-ID: <20110608203615.GB13151@n2100.arm.linux.org.uk> (raw)
In-Reply-To: <20110608202323.GA4430@e102144-lin.cambridge.arm.com>

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.

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.

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