Re: [PATCH 08/20] powerpc: Preemptible mmu_gather

Benjamin Herrenschmidt
2010-08-31T06:32:30Z

On Tue, 2010-08-31 at 16:26 +1000, Benjamin Herrenschmidt wrote:
> On Sat, 2010-08-28 at 16:16 +0200, Peter Zijlstra wrote:
> > Fix up powerpc to the new mmu_gather stuffs.
>
> Unfortunately, I think this is broken...
>
> First there's an actual bug here:
>
> > last = _switch(old_thread, new_thread);
> >
> > +#ifdef CONFIG_PPC64
> > + if (task_thread_info(new)->local_flags & _TLF_LAZY_MMU) {
> > + task_thread_info(new)->local_flags &= ~_TLF_LAZY_MMU;
> > + batch = &__get_cpu_var(ppc64_tlb_batch);
> > + batch->active = 1;
> > + }
> > +#endif
> > +
>
> Here, you are coming out of _switch() which will have swapped the
> stack and non-volatile registers to the state they were in when the
> new task was originally switched-out. Thus "new" which is a local variable
> (either on stack or in a non-volatile register) will now refer to whatever
> was the next task back then.
>
> I suppose that's what's causing the similar patch you have in -rt to
> fail btw. This could be fixed easily by using "current" instead.

And here's a fix. Using current_thread_info() everywhere is better
anyways, you are not supposed to touch the local_flags within anything
but "current" so we may as well make it explicit.

Feel free to fold that into your -rt patch and this one if you decide
not to drop that whole part.

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 6ee4b09..6e8b6de 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -517,12 +517,12 @@ struct task_struct *__switch_to(struct task_struct *prev,

batch = &__get_cpu_var(ppc64_tlb_batch);
if (batch->active) {
- task_thread_info(prev)->local_flags |= _TLF_LAZY_MMU;
+ current_thread_info()->local_flags |= _TLF_LAZY_MMU;
if (batch->index)
__flush_tlb_pending(batch);
batch->active = 0;
}
-#endif
+#endif /* CONFIG_PPC64 */

local_irq_save(flags);

@@ -539,12 +539,12 @@ struct task_struct *__switch_to(struct task_struct *prev,
last = _switch(old_thread, new_thread);

#ifdef CONFIG_PPC64
- if (task_thread_info(new)->local_flags & _TLF_LAZY_MMU) {
- task_thread_info(new)->local_flags &= ~_TLF_LAZY_MMU;
+ if (current_thread_info()->local_flags & _TLF_LAZY_MMU) {
+ current_thread_info()->local_flags &= ~_TLF_LAZY_MMU;
batch = &__get_cpu_var(ppc64_tlb_batch);
batch->active = 1;
}
-#endif
+#endif /* CONFIG_PPC64 */

local_irq_restore(flags);