[PATCH] 8xx: get_mmu_context() for (very) FEW_CONTEXTS and KERNEL_PREEMPT race/starvation issue

[Eugene Surovegin]

On Thu, Jun 30, 2005 at 09:26:07AM +1000, Benjamin Herrenschmidt wrote:
> 
> > Execution is resumed exactly where it has been interrupted.
> > 
> > > The idea behind my patch was to get rid of that nr_free_contexts counter 
> > > that is (I thing) redundant with the context_map.
> > 
> > Apparently its there to avoid the spinlock exactly on !FEW_CONTEXTS machines.
> > 
> > I suppose that what happens is that get_mmu_context() gets preempted after stealing
> > a context (so nr_free_contexts = 0), but before setting next_mmu_context to the 
> > next entry
> > 
> > next_mmu_context = (ctx + 1) & LAST_CONTEXT;
> 
> Ugh ? Can switch_mm() be preempted at all ? Did I miss yet another
> "let's open 10 gazillion races for gun" Ingo patch ?

No, it can't. schedule() disables preemption at the very beginning.

-- 
Eugene