Re: Semantics of smp_mb() [was : Re: [PATCH] Fix RCU race in access of nohz_cpu_mask ]

[Keith Owens]

On Tue, 13 Dec 2005 08:20:27 -0800, 
"Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
>If the variable p references MMIO rather than normal memory, then
>wmb() and rmb() are needed instead of smp_wmb() and smp_rmb().

mmiowb(), not wmb().  IA64 has a different form of memory fence for I/O
space compared to normal memory.  MIPS also has a non-empty form of
mmiowb().

>This is because the I/O device needs to see the accesses ordered
>even in a UP system.

Why does mmiowb() map to empty on most systems, including Alpha?
Should it not map to wmb() for everything except IA64 and MIPS?

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