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

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Date:	Wed, 4 Sep 2013 14:54:04 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Will Deacon <will.deacon@....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, 4 Sep 2013, Will Deacon wrote:

> Hmm, why can't you get interrupted during atomic64_xchg? On ARM, we have the
> following sequence:

AFAICT atomic means one uninterruptible action.

>   static inline u64 atomic64_xchg(atomic64_t *ptr, u64 new)
>   {
>   	u64 result;
>   	unsigned long tmp;
>
>   	smp_mb();
>
>   	__asm__ __volatile__("@ atomic64_xchg\n"
>   "1:	ldrexd	%0, %H0, [%3]\n"
>   "	strexd	%1, %4, %H4, [%3]\n"
>   "	teq	%1, #0\n"
>   "	bne	1b"
>   	: "=&r" (result), "=&r" (tmp), "+Qo" (ptr->counter)
>   	: "r" (&ptr->counter), "r" (new)
>   	: "cc");
>
>   	smp_mb();
>
>   	return result;
>   }
>
> which relies on interrupts clearing the exclusive monitor to force us back
> around the loop in the inline asm. I could imagine other architectures doing
> similar, but only detecting the other writer if it used the same
> instructions.

Well I have never done ARM asm but this looks vaguely like a cmpxchg loop?
That would either perform an atomic change or fail and retry?
If so it still fits the definition of atomic. The change or fail operation
is atomic.


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