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SubjectRe: Is clobber "memory" in include/asm-i386/system.h necessary?


On Sat, 4 May 1996, Warner Losh wrote:
>
> : "volatile" itself causes suboptimal code to be generated
> : even in places where it's unnecessary, such as between cli() and sti().
>
> What about MP machines? Wouldn't volatile still be needed between a
> cli() and sti() because other processors might change the value out
> from under you? Granted, a higher level of locking would prevent
> that, but there are cases where you might want that to happen (like in
> the lock manager code, assuming it uses atomic memory access
> instructions).

"volatile" is _never_ a good idea, even in SMP. You already touched on
why it's not a good idea: it's unnecessary if we have some locking to
prevent concurrent accesses.

Now, if we DON'T have locking that prevents concurrent accesses, then we
have a major bug in the system.

Ergo, using "volatile" means that we have a major bug in the system.

QED.

Linus


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