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

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Message-id: <alpine.LFD.2.00.0811070256450.13034@xanadu.home>
Date:	Fri, 07 Nov 2008 03:12:18 -0500 (EST)
From:	Nicolas Pitre <nico@....org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

On Thu, 6 Nov 2008, Andrew Morton wrote:

> 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?

Please do me a favor and look for those very unfrequent posts I've sent 
to lkml lately.  I've explained it all at least 3 times so far, to Peter 
Zijlstra, to David Howells, to Mathieu Desnoyers, and now to you.

> 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.

Yes!  And this is so on *purpose*.  Please take some time to read the 
comment that goes along with it, and if you're still not convinced then 
look for those explanation emails I've already posted.

> /*
>  * Caller must provide locking to protect *caller_state
>  */

NO!  This is meant to be LOCK FREE!


Nicolas
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