Re: Question about memory barriers

[linux-os (Dick Johnson)]

On Thu, 2 Feb 2006, Alan Stern wrote:

> The kernel's documentation about memory barriers is rather skimpy.  I
> gather that rmb() guarantees that all preceding reads will have completed
> before any following reads are made, and wmb() guarantees that all
> preceding writes will have completed before any following writes are made.
> I also gather that mb() is essentially the same as rmb() and wmb() put
> together.
>
> But suppose I need to prevent a read from being moved past a write?  It
> doesn't look like either rmb() or wmb() will do this.  And if mb() is the
> same as "rmb(); wmb();" then it won't either.  So what's the right thing
> to do?
>
> Alan Stern

If you use the correct macros for device I/O (in other words
the operations are upon volatile objects), there can never
be any re-ordering of any associated code.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.66 BogoMips).
Warning : 98.36% of all statistics are fiction.
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