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

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Message-ID: <20081107204546.GA3324@Krystal>
Date:	Fri, 7 Nov 2008 15:45:46 -0500
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	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>,
	linux-arch@...r.kernel.org
Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

* Mathieu Desnoyers (mathieu.desnoyers@...ymtl.ca) wrote:
> * Peter Zijlstra (a.p.zijlstra@...llo.nl) wrote:
> > On Fri, 2008-11-07 at 14:18 -0500, Mathieu Desnoyers wrote:
> > > * Steven Rostedt (rostedt@...dmis.org) wrote:
> > > > 
> > > > On Fri, 7 Nov 2008, Mathieu Desnoyers wrote:
> > > > > 
> > > > >   __m_cnt_hi
> > > > >  is read before
> > > > >   mmio cnt_lo read
> > > > > 
> > > > > for the detailed reasons explained in my previous discussion with
> > > > > Nicolas here :
> > > > > http://lkml.org/lkml/2008/10/21/1
> > > > > 
> > > > > I use smp_rmb() to do this on SMP systems (hrm, actually, a rmb() could
> > > > > be required so it works also on UP systems safely wrt interrupts).
> > > > 
> > > > smp_rmb turns into a compiler barrier on UP and should prevent the below
> > > > description.
> > > > 
> > > 
> > > Ah, right, preserving program order on UP should be enough. smp_rmb()
> > > then.
> > 
> > 
> > I'm not quite sure I'm following here. Is this a global hardware clock
> > you're reading from multiple cpus, if so, are you sure smp_rmb() will
> > indeed be enough to sync the read?
> > 
> > (In which case the smp_wmb() is provided by the hardware increasing the
> > clock?)
> > 
> > If these are per-cpu clocks then even in the smp case we'd be good with
> > a plain barrier() because you'd only ever want to read your own cpu's
> > clock (and have a separate __m_cnt_hi per cpu).
> > 
> > Or am I totally missing out on something?
> > 
> 
> This is the global hardware clock scenario.
> 
> We have to order an uncached mmio read wrt a cached variable read/write.
> The uncached mmio read vs smp_rmb() barrier (e.g. lfence instruction)
> should be insured by program order because the read will skip the cache
> and go directly to the bus. Luckily we only do a mmio read and no mmio
> write, so mmiowb() is not required.
> 
> You might be right in that it could require more barriers.
> 
> Given adequate program order, we can assume the the mmio read will
> happen "on the spot", but that the cached read may be delayed.
> 
> What we want is :
> 
> readl(io_addr)
> read __m_cnt_hi
> write __m_cnt_hi
> 
> With the two reads in the correct order. If we consider two consecutive
> executions on the same CPU :
> 
> readl(io_addr)
> read __m_cnt_hi
> write __m_cnt_hi
> 
> readl(io_addr)
> read __m_cnt_hi
> write __m_cnt_hi
> 
> We might have to order the read/write pair wrt the following readl, such
> as :
> 
> smp_rmb();  /* Waits for every cached memory reads to complete */
> readl(io_addr);
> barrier();  /* Make sure the compiler leaves mmio read before cached read */
> read __m_cnt_hi
> write __m_cnt_hi
> 
> smp_rmb();  /* Waits for every cached memory reads to complete */
> readl(io_addr)
> barrier();  /* Make sure the compiler leaves mmio read before cached read */
> read __m_cnt_hi
> write __m_cnt_hi
> 
> Would that make more sense ?
> 

Oh, actually, I got things reversed in this email : the readl(io_addr)
must be done _after_ the __m_cnt_hi read.

Therefore, two consecutive executions would look like :

barrier();  /* Make sure the compiler does not reorder __m_cnt_hi and
               previous mmio read. */
read __m_cnt_hi
smp_rmb();  /* Waits for every cached memory reads to complete */
readl(io_addr);
write __m_cnt_hi


barrier();  /* Make sure the compiler does not reorder __m_cnt_hi and
               previous mmio read. */
read __m_cnt_hi
smp_rmb();  /* Waits for every cached memory reads to complete */
readl(io_addr);
write __m_cnt_hi

Mathieu

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

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