Re: (*(unsigned long *)&jiffies)++;

[Davide Libenzi]

On Thu, 06 Jan 2000, Richard B. Johnson wrote:
> 1	movl foo,%eax            ! Get it
> 2	leal 1(%eax),%edx        ! bump it by one
> 3	movl %edx,foo            ! write it back
> 4	movl foo,%eax            ! Then do something dumb
> 5	movl foo,%edx            ! Then do something dumb again
> 6	pushl %edx
> 7	pushl $.LC0
> 8	call printf
> 9	addl $8,%esp

This is the correct behaviour the code must follow.
Even if it has just stored the value in foo ( 3 ) and even it can think that
the value is %edx is the last one, the volatile keyword force a reload in 5.
The step 4 is not clear for me !
IMVHO doing a jiffies++ or (*(unsigned long *)&jiffies)++ is exactly the same
thing in this case.
I mean that :

incl	jiffies

or

movl	jiffies, %eax
incl	%eax
movl	%eax, jiffies

does not introduce worse issues in SMP.
Sure the volatile keyword avoid the compiler to optimize :

movl	jiffies, %eax
incl	%eax

...

movl	%eax, jiffies

As a general rule I suggest to introduce any tricks that can solve speed or
code optimization issues where there's the need, avoiding them in caseless
situations.

Cheers,
	Davide.

-- 
"Debian, the freedom in freedom."