atomic[64]_[read|set] asm implementations

In the fallout from the recent atomic_t volatility discussions, patches have been posted to moot the compiler correctness issues by implementing atomic[64]_[read|set] in inline assembly on powerpc, i386, and x86_64. While I personally don't consider such implementations to be critically necessary, they slightly reduce binary size and greatly reduce lkml controversy, so I like them.

I'm not an inline assembly expert, but I would welcome it if those who are would like to submit implementations of these functions for architectures they're intimately familiar with. I'll pull them into the atomic_t patch set I've stumbled into shepherding, and hopefully we can commit something all at once that makes all architectures consistent, resists compiler bugs, and doesn't piss off too many people.

For architectures whose maintainers aren't worried and whose developers/users aren't bothered enough to submit an inline assembly patch, I'll just keep the inlines with the *(volatile foo *)& casts, unless of course ISO clarifies the C standard to different effect while we're arguing over semantics and micro-optimizations.

-- Chris

I have no problem with your latest patch for ARM. Moving ARM to assembly will actually remove some information which the compiler has when it knows about the access to be performed.

If atomic_read() were converted to inline assembly, we'd prevent the compiler from knowing that a load has been performed and it should therefore avoid using the result of the load where possible for the following few instructions (due to result delays.)

Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of: