linux-kernel - Re: [Xen-devel] [PATCH 2/2] x86/pae: use 64 bit atomic xchg function in native_ptep_get_and

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Message-Id: <5B7A7E6C02000078001DFD15@prv1-mh.provo.novell.com>
Date:   Mon, 20 Aug 2018 02:40:12 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Juergen Gross" <jgross@...e.com>
Cc:     "the arch/x86 maintainers" <x86@...nel.org>, <tglx@...utronix.de>,
        "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>, <mingo@...hat.com>,
        <linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH 2/2] x86/pae: use 64 bit atomic xchg
 function in native_ptep_get_and_clear

>>> On 20.08.18 at 07:14, <jgross@...e.com> wrote:
> @@ -148,14 +150,14 @@ static inline void pud_clear(pud_t *pudp)
>  #ifdef CONFIG_SMP
>  static inline pte_t native_ptep_get_and_clear(pte_t *ptep)
>  {
> -	pte_t res;
> +	union {
> +		pte_t pte;
> +		long long val;
> +	} res;

Why the union? pte_t already is one, with the pte field being what
you're after ...

> -	/* xchg acts as a barrier before the setting of the high bits */
> -	res.pte_low = xchg(&ptep->pte_low, 0);
> -	res.pte_high = ptep->pte_high;
> -	ptep->pte_high = 0;
> +	res.val = arch_atomic64_xchg((atomic64_t *)ptep, 0);

... here.

Jan