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

[Jamie Lokier]

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
region.

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


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [EMAIL PROTECTED]
Please read the FAQ at http://www.tux.org/lkml/