Re: spin_lock forgets to clobber memory and other smp fixes [was Re: [patch] waitqueue optimization, 2.4.0-test7]

From: Jamie Lokier (
Date: Thu Sep 07 2000 - 11:55:59 EST

Linus Torvalds wrote:
> Change it to something like
> __asm__("":"=r" (x):"0" (x));
> and the "volatile" should matter.

Yes it does. Without "volatile", the asm disappears :-)

> Not for memory references, perhaps. But for the movement issues.

The compiler isn't moving memory references around "asm volatile", but
it is doing CSE around them to _eliminate_ memory references.

Thus spin_lock needs the memory clobber, to prevent CSE of non-volatile
memory references between the critical region and outside the critical

Maybe spin_unlock doesn't need one because CSE doesn't work the other
way. (I'd put that clobber in anyway, to be sure).

-- Jamie

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