linux-kernel - Re: [PATCH v2] clarify usage expectations for cnt32_to

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Date:	Sun, 9 Nov 2008 01:48:55 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Nicolas Pitre <nico@....org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Russell King <rmk+lkml@....linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	lkml <linux-kernel@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>, benh@...nel.crashing.org,
	paulus@...ba.org, David Miller <davem@...emloft.net>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH v2] clarify usage expectations for cnt32_to_63()

* Nicolas Pitre (nico@....org) wrote:
> Currently, all existing users of cnt32_to_63() are fine since the CPU
> architectures where it is used don't do read access reordering, and user
> mode preemption is disabled already.  It is nevertheless a good idea to
> better elaborate usage requirements wrt preemption, and use an explicit
> memory barrier on SMP to avoid different CPUs accessing the counter
> value in the wrong order.  On UP a simple compiler barrier is 
> sufficient.
> 
> Signed-off-by: Nicolas Pitre <nico@...vell.com>
> ---
> 
...
> @@ -68,9 +77,10 @@ union cnt32_to_63 {
>   */
>  #define cnt32_to_63(cnt_lo) \
>  ({ \
> -	static volatile u32 __m_cnt_hi; \
> +	static u32 __m_cnt_hi; \

It's important to get the smp_rmb() here, which this patch provides, so
consider this patch to be acked-by me. The added documentation is needed
too.

But I also think that declaring the static u32 __m_cnt_hi here is
counter-intuitive for developers who wish to use it.

I'd recommend letting the declaration be done outside of cnt32_to_63 so
the same variable can be passed as parameter to more than one execution
site.

Mathieu


>  	union cnt32_to_63 __x; \
>  	__x.hi = __m_cnt_hi; \
> + 	smp_rmb(); \
>  	__x.lo = (cnt_lo); \
>  	if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
>  		__m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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