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

[Paul E. McKenney]

On Thu, Dec 15, 2005 at 11:46:26PM -0800, Jeremy Higdon wrote:
> Roland Dreier got this right.  The purpose of the mmiowb is
> to ensure that writes to I/O devices while holding a spinlock
> are ordered with respect to writes issued after the original
> processor releases and a second processor acquires said
> spinlock.
> 
> A MMIO read would be sufficient, but is much heavier weight.
> 
> On the SGI MIPS-based systems, the "sync" instruction was used.
> On the Altix systems, a register on the hub chip is read.
> 
> >From comments by jejb, we're looking at modifying the mmiowb
> API by adding an argument which would be a register to read
> from if the architecture in question needs ordering in this
> way but does not have a lighter weight mechanism like the Altix
> mmiowb.  Since there will now need to be a width indication,
> mmiowb will be replaced with mmiowb[bwlq].

Any progress on this front?  I figured that I would wait to update
the ordering document until after this change happened, but if it
is going to be awhile, I should proceed with the current API.

Thoughts?

						Thanx, Paul
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