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Re: thoughts on libatomic

On 04/23/2012 03:29 PM, Richard Henderson wrote:

- load_n.c:
   - I'm concerned about the CAS on read-only mprotected pages?
     Why again do we think this is safe?  Does the standard explicitly
    allow this?  Or should we just use a lock in this case?
Andrew, you had a bit of back-and-forth with someone about this.
Can you dig that up?

yes, this keeps coming up again and again.... I think you should take it up with Lawrence Crowl... He was the one, (along with some input from Jeffrey Yasskin) that concluded that it was OK to use it, even if it was just sort of shoehorned in so that we could get 16 byte lock free on the most common architecture...

it came down to something like the architecture manual entry for cmpxchg16 states that a store cycle may be added by the hardware under the covers, and as such, it is a part of the basic machine description, and therefore we might as well use the instruction even though we may be adding the store cycle ourselves sometimes....

I haven't found the actual communication for reference yet, I'll look again tomorrow.


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