Messages in this thread | | | Subject | Re: Question about memory barriers | From | Roland Dreier <> | Date | Thu, 02 Feb 2006 13:37:48 -0800 |
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>>>>> "Alan" == Alan Stern <stern@rowland.harvard.edu> writes:
Alan> The kernel's documentation about memory barriers is rather Alan> skimpy. I gather that rmb() guarantees that all preceding Alan> reads will have completed before any following reads are Alan> made, and wmb() guarantees that all preceding writes will Alan> have completed before any following writes are made. I also Alan> gather that mb() is essentially the same as rmb() and wmb() Alan> put together.
Most of this is correct, except that mb() is stronger than just rmb() and wmb() put together. All memory operations before the mb() will complete before any operations after the mb(). A better way to understand this is to look at the sparc64 definition:
#define mb() \ membar_safe("#LoadLoad | #LoadStore | #StoreStore | #StoreLoad")
Alan> But suppose I need to prevent a read from being moved past a Alan> write? It doesn't look like either rmb() or wmb() will do Alan> this. And if mb() is the same as "rmb(); wmb();" then it Alan> won't either. So what's the right thing to do?
As described above, mb() will work in this case. It actually guarantees more than you need, so you could conceivably define a new primitive, but the current barriers are hard enough for people to figure out ;)
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