linux-kernel - Re: [PATCH v11 1/5] xen: move xen_setup_runstate_info and get_runstate

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Date:	Thu, 5 Nov 2015 16:48:20 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	xen-devel@...ts.xensource.com, linux@....linux.org.uk,
	Ian Campbell <ian.campbell@...rix.com>, arnd@...db.de,
	marc.zyngier@....com, catalin.marinas@....com,
	konrad.wilk@...cle.com, will.deacon@....com,
	linux-kernel@...r.kernel.org, olof@...om.net,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v11 1/5] xen: move xen_setup_runstate_info and
 get_runstate_snapshot to drivers/xen/time.c

Hi,

> +static u64 get64(const u64 *p)
> +{
> +	u64 ret;
> +
> +	if (BITS_PER_LONG < 64) {
> +		u32 *p32 = (u32 *)p;
> +		u32 h, l;
> +
> +		/*
> +		 * Read high then low, and then make sure high is
> +		 * still the same; this will only loop if low wraps
> +		 * and carries into high.
> +		 * XXX some clean way to make this endian-proof?
> +		 */
> +		do {
> +			h = p32[1];
> +			barrier();
> +			l = p32[0];
> +			barrier();
> +		} while (p32[1] != h);

I realise this is simply a move of existing code, but it may be better
to instead have:

do {
	h = READ_ONCE(p32[1]);
	l = READ_ONCE(p32[0]);
} while (READ_ONCE(p32[1] != h);

Which ensures that each load is a single access (though it almost
certainly would be anyway), and prevents the compiler from having to
reload any other memory locations (which the current barrier() usage
forces).

> +
> +		ret = (((u64)h) << 32) | l;
> +	} else
> +		ret = *p;

Likewise, this would be better as READ_ONCE(*p), to force a single
access.

> +
> +	return ret;
> +}

> +	do {
> +		state_time = get64(&state->state_entry_time);
> +		barrier();
> +		*res = *state;
> +		barrier();

You can also have:

	*res = READ_ONCE(*state);

That will which will handle the barriers implicitly.

Thanks,
Mark.

> +	} while (get64(&state->state_entry_time) != state_time);
> +}
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