linux-kernel - Re: [PATCH 08/20] powerpc: Preemptible mmu

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Date:	Tue, 31 Aug 2010 16:31:05 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...e.de>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Yanmin Zhang <yanmin_zhang@...ux.intel.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH 08/20] powerpc: Preemptible mmu_gather

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);
 


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