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ACCESS_ONCE()

ACCESS_ONCE()

Posted Aug 6, 2012 12:05 UTC (Mon) by tvld (guest, #59052)
In reply to: ACCESS_ONCE() by PaulMcKenney
Parent article: ACCESS_ONCE()

I don't interpret 1.10.24 the same way. To me, it just ensures the compiler that there cannot be an infinite loop without any synchronization or side-effects in it. The only purpose of that that I see is similar to the note in 1.10.24 -- removal of empty loops (e.g., after stuff has been hoisted out of the loop).
I don't see how it would restrict reads-from inside of infinite loops. Even if the relaxed loads would have to stay in the loop, they'd still be allowed to read from an "old" write. (That's why I mentioned 1.10.25, which could mean that they eventually should pick up a recent value, forever; in that case, the load would also have to stay in the loop).
Why are you reading 1.10.24 differently? What's the detailed reasoning?

The second issue, silently reloading from memory for a mo_relaxed load, would not be allowed, I think. The abstract machine wouldn't do it. It is allowed under as-if for nonatomic accesses (with no synchronization in-between etc.) because we can assume data-race-freedom and thus no concurrent modification by other threads.


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ACCESS_ONCE()

Posted Aug 6, 2012 16:58 UTC (Mon) by PaulMcKenney (subscriber, #9624) [Link]

Suppose that a loop depends on a relaxed atomic load. Suppose that the compiler unrolls the loop by (say) a factor of two. I don't see anything in 1.10.24 that tells me that the compiler is prohibited from actually performing the relaxed atomic load only once per iteration of the unrolled loop, that is to say, only once per two iterations of the loop as written in the source code. Such an optimization could be a problem in some situations.

Don't get me wrong, I would be happy to learn that I am being overly paranoid, and that something actually forces the compiler to actually perform each and every relaxed atomic load, but I have heard too many compiler writers discussing optimizations that are not consistent with this less-paranoid view of this matter.

Your argument that silent reloading from memory for a memory_order_relaxed load does sound more convincing. If I consistently hear the same thing from enough compiler writers over a sufficiently long period of time, I will start recommending ACCESS_ONCE() only to prevent the compiler from optimizing atomic loads out, not to prevent the compiler from reloading from atomics.


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