Re: [patch] spinlocks: remove 'volatile'

[linux-os (Dick Johnson)]

On Fri, 7 Jul 2006, Linus Torvalds wrote:

>
>
> On Fri, 7 Jul 2006, linux-os (Dick Johnson) wrote:
>>
>> Now Linus declares that instead of declaring an object volatile
>> so that it is actually accessed every time it is referenced, he wants
>> to use a GNU-ism with assembly that tells the compiler to re-read
>> __every__ variable existing im memory, instead of just one. Go figure!
>
> Actually, it's not just me.
>
> Read things like the Intel CPU documentation.
>
> IT IS ACTIVELY WRONG to busy-loop on a variable. It will make the CPU
> potentially over-heat, causing degreaded performance, and you're simply
> not supposed to do it.

This is a bait and switch argument. The code was displayed to show
the compiler output, not an example of good coding practice.

Furthermore, I'm still waiting for the new spin-friction that's
supposed to cause the increased heat. It doesn't exist and furthermore
no Intel processor instruction manual (that is available for public
inspection) claims that busy-waiting increases any heating, only that
some processors that __can__ fall back into a low-power mode will not
do so if they are spinning (naturally). But this is just another
bait-and-switch as well because I only wrote about volatile and
I even provided references.

>
>> Reference:
>> /usr/src/linux-2.6.16.4/include/linux/compiler-gcc.h:
>> #define barrier() __asm__ __volatile__("": : :"memory")
>
> Actually, for your kind of stupid loop, you should use
>
> - include/asm-i386/processor.h:
> 	#define cpu_relax()        rep_nop()
>

Again, I didn't propose to do that. In fact, your spin-lock
code already inserts "rep nops" and I never implied that they
should be removed. I said only that "volatile" still needs to
be used, not some macro that tells the compiler that everything
in memory probably got trashed. Read what I said, not what you
think some idiot might have said.

> where rep_nop() is
>
> 	/* REP NOP (PAUSE) is a a good thing to insert into busy-wait loops. */
> 	static inline void rep_nop(void)
> 	{
> 		__asm__ __volatile__("rep;nop": : :"memory");
> 	}
>
> on x86, and can be other things on other CPU's. On ppc64 it is
>
> 	#define cpu_relax()     do { HMT_low(); HMT_medium(); barrier(); } while (0)
>
> where those HMT macros adjust thread priority.
>

Not either of these things have anything to do with the topic and
I never even implied that they did. Again, I spoke only of the
well known volatile keyword. Nothing else.

> In other words, you just don't know what you're talking about. "volatile"
> is simply not useful, and the fact that you cannot seem to grasp that is
> _your_ problem.
>

Well, in fact I do know what I'm talking about. Your bait-and-switch
arguments just won't work with me.

> Repeat after me (or just shut up about things that you not only don't know
> about, but are apparently not willing to even learn):
>
> 	"'volatile' is useless. The things it did 30 years ago are much
> 	 more complex these days, and need to be tied to much more
> 	 detailed rules that depend on the actual particular problem,
> 	 rather than one keyword to the compiler that doesn't actually
> 	 give enough information for the compiler to do anything useful"
>
> Comprende?
>
> 		Linus
>

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.4 on an i686 machine (5592.89 BogoMips).
New book: http://www.AbominableFirebug.com/
_


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