linux-kernel - Re: [gcv v3 27/35] arm: Replace __get_cpu

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Date:	Wed, 4 Sep 2013 19:21:13 +0100
From:	Will Deacon <will.deacon@....com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Tejun Heo <tj@...nel.org>,
	"akpm@...uxfoundation.org" <akpm@...uxfoundation.org>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Steven Rostedt <srostedt@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [gcv v3 27/35] arm: Replace __get_cpu_var uses

On Wed, Sep 04, 2013 at 07:09:04PM +0100, Christoph Lameter wrote:
> On Wed, 4 Sep 2013, Will Deacon wrote:
> 
> > On ARM, yes. I'm worried that there may be an architecture where the change-
> > or-fail operation would only fail if the access from the interrupt handler
> > *also* used that change-or-fail instruction, which isn't the case with
> > this_cpu_inc.
> >
> > I have no idea if such an architecture exists :)
> 
> Atomic operations use atomic_t. this_cpu operations can only use regular
> scalars. So the set of variables that are updated by each should be
> distinct.

Right, except that your patch contained the following hunk:

Index: linux/arch/arm64/kernel/debug-monitors.c
===================================================================
--- linux.orig/arch/arm64/kernel/debug-monitors.c       2013-08-26 13:48:40.956794980 -0500
+++ linux/arch/arm64/kernel/debug-monitors.c    2013-08-26 13:48:40.952795024 -0500
@@ -98,11 +98,11 @@ void enable_debug_monitors(enum debug_el

        WARN_ON(preemptible());

-       if (local_inc_return(&__get_cpu_var(mde_ref_count)) == 1)
+       if (this_cpu_inc_return(mde_ref_count) == 1)
                enable = DBG_MDSCR_MDE;

        if (el == DBG_ACTIVE_EL1 &&
-           local_inc_return(&__get_cpu_var(kde_ref_count)) == 1)
+           this_cpu_inc_return(kde_ref_count) == 1)
                enable |= DBG_MDSCR_KDE;

        if (enable && debug_enabled) {


Then we have:

#define local_inc_return(l) atomic_long_inc_return(&(l)->a)

static inline long atomic_long_inc_return(atomic_long_t *l)
{
	atomic_t *v = (atomic_t *)l;

	return (long)atomic_inc_return(v);
}


So that casting lets the two interfaces overlap (and indeed they do after
your patch, since local_dec_and_test is still used to the same variable).

Will
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