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

Nicolas Pitre
2008-11-07T20:37:26Z

On Fri, 7 Nov 2008, Mathieu Desnoyers wrote:

> I want to make sure
>
> __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).
>
> 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)

Could you have hardware doing such things? You would get a non cached
and more expensive read on CPU B which is not in program order with the
read that should have happened before, and before that second out of
order read could be performed, you'd have a full sequence in program
order performed on CPU A.

Nicolas