[BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Fri, May 27, 2011 at 02:06:29PM +0200, Ingo Molnar wrote:
> The expectations are to have irqs off (we are holding the runqueue 
> lock if !__ARCH_WANT_INTERRUPTS_ON_CTXSW), so that's not workable i 
> suspect.

Just a thought, but we _might_ be able to avoid a lot of this hastle if
we had a new arch hook in finish_task_switch(), after finish_lock_switch()
returns but before the old MM is dropped.

For the new ASID-based switch_mm(), we currently do this:

1. check ASID validity
2. flush branch predictor
3. set reserved ASID value
4. set new page tables
5. set new ASID value

This will be shortly changed to:

1. check ASID validity
2. flush branch predictor
3. set swapper_pg_dir tables
4. set new ASID value
5. set new page tables

We could change switch_mm() to only do:

1. flush branch predictor
2. set swapper_pg_dir tables
3. check ASID validity
4. set new ASID value

At this point, we have no user mappings, and so nothing will be using the
ASID at this point.  Then in a new post-finish_lock_switch() arch hook:

5. check whether we need to do flushing as a result of ASID change
6. set new page tables

I think this may simplify the ASID code.  It needs prototyping out,
reviewing and testing, but I think it may work.

And I think it may also be workable with the CPUs which need to flush
the caches on context switches - we can postpone their page table
switch to this new arch hook too, which will mean we wouldn't require
__ARCH_WANT_INTERRUPTS_ON_CTXSW on ARM at all.

Any thoughts (if you've followed what I'm going on about) ?