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

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Date:	Fri, 7 Nov 2008 14:18:34 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	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>,
	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>,
	linux-arch@...r.kernel.org
Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

* 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.

Thanks,

Mathieu

> -- Steve
> 
> > 
> > The write side is between the hardware counter, which is assumed to
> > increment monotonically between each read, and the value __m_cnt_hi
> > updated by the CPU. I don't see where we could put a wmb() there.
> > 
> > Without barrier, the smp race looks as follow :
> > 
> > 
> > CPU    A                                    B
> >                                             read hw cnt low (0xFFFFFFFA)
> >        read __m_cnt_hi (0x80000000)
> >        read hw cnt low (0x00000001)
> >        (wrap detected :
> >         (s32)(0x80000000 ^ 0x1) < 0)
> >        write __m_cnt_hi = 0x00000001
> >                                             read __m_cnt_hi (0x00000001)
> >        return 0x0000000100000001
> >                                             (wrap detected :
> >                                              (s32)(0x00000001 ^ 0xFFFFFFFA) < 0)
> >                                             write __m_cnt_hi = 0x80000001
> >                                             return 0x80000001FFFFFFFA
> >                                                                   (time jumps)
> > 
> > And UP interrupt race :
> > 
> >        Thread context                       Interrupts
> >        read hw cnt low (0xFFFFFFFA)
> >                                             read __m_cnt_hi (0x80000000)
> >                                             read hw cnt low (0x00000001)
> >                                             (wrap detected :
> >                                              (s32)(0x80000000 ^ 0x1) < 0)
> >                                             write __m_cnt_hi = 0x00000001
> >        read __m_cnt_hi (0x00000001)
> >                                             return 0x0000000100000001
> >        (wrap detected :
> >         (s32)(0x00000001 ^ 0xFFFFFFFA) < 0)
> >        write __m_cnt_hi = 0x80000001
> >        return 0x80000001FFFFFFFA
> >                 (time jumps)
> > 

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