Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

Mathieu Desnoyers
2008-11-07T19:18:47Z

* Steven Rostedt (rostedt@goodmis.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