linux-kernel - Re: [RFC patch 08/18] cnt32_to_63 should use smp

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Date:	Thu, 6 Nov 2008 22:05:30 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, Nicolas Pitre <nico@....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: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

On Fri, 07 Nov 2008 00:23:44 -0500 Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

>  #define cnt32_to_63(cnt_lo) \
>  ({ \
> -	static volatile u32 __m_cnt_hi; \
> +	static u32 __m_cnt_hi; \
>  	union cnt32_to_63 __x; \
>  	__x.hi = __m_cnt_hi; \
> +	smp_rmb(); 	/* read __m_cnt_hi before mmio cnt_lo */ \
>  	__x.lo = (cnt_lo); \
>  	if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
>  		__m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \

Oh dear.  We have a macro which secretly maintains
per-instantiation-site global state?  And doesn't even implement locking
to protect that state?

I mean, the darned thing is called from sched_clock(), which can be
concurrently called on separate CPUs and which can be called from
interrupt context (with an arbitrary nesting level!) while it was running
in process context.

Who let that thing into Linux?


Look:

/*
 * Caller must provide locking to protect *caller_state
 */
u32 cnt32_to_63(u32 *caller_state, u32 cnt_lo);

But even that looks pretty crappy.
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