Re: [patch] spinlocks: remove 'volatile'

[Krzysztof Halasa]

Chase Venters <chase.venters@xxxxxxxxxxxx> writes:

> You're mincing my words. The reason "memory" is on the clobber list is
> because 
> a lock is supposed to synchronize all memory accesses up to that point. It's 
> a fence / a barrier, because if the compiler re-orders your loads/stores 
> across the lock, you're in trouble. That's exactly what I was pointing out.

Sure, but a barrier alone isn't enough. You have to use assembler and
it's beyond scope of C volatile.

> A volatile cast lets you prevent the compiler from always treating the 
> variable as volatile.

Yes, if that's what you really want.

>> If the "volatile" is used the wrong way (which is probably true for most
>> cases), then volatile cast and barrier() will be wrong as well. You need
>> locks or atomic access, meaningful on hardware level.
>
> No. Linus already described what some example invalid uses of "volatile"
> are. 
> One example is the very one this whole thread is about - using 'volatile' on 
> the declaration of the spinlock counter. That usage is _wrong_, and
> barrier() 
> would not be.

That's a special case, because you want to invalidate all variables,
but you still need locking. I.e., barrier() alone doesn't buy you
anything WRT to hardware.

> Volatile originally existed to tell the compiler a variable could change at 
> will. Because of reordering, it's almost never sufficient with our modern 
> compilers and CPUs. That's precisely where barrier() (and/or its hardware 
> equivalents) help in places where 'volatile' is wrong.

How does barrier() help here? Some example, maybe?
What do you consider a barrier() hardware equivalent?
Don't you think you're mixing compiler optimization and operation of
the hardware?

> Your statement is 
> additionally wrong because one use-case of memory barriers is to safely
> write 
> lock-free code.

You can't safely write lock-free code in C, if you have to deal
with hardware or SMP. C don't know about hardware.
-- 
Krzysztof Halasa
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