Re: Is clobber "memory" in include/asm-i386/system.h necessary?
Linus Torvalds (torvalds@cs.helsinki.fi)
Mon, 6 May 1996 21:05:42 +0300 (EET DST)
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