Re: atomic counters larger than 24 bits?

[Jeremy Fitzhardinge <jeremy@goop.org>]

Dan Magenheimer wrote:
> In tmem, I have some counters that I need to keep coherent,
> preferably without a lock, so I'm using atomic_inc/dec etc.
> However the comments in include/asm-x86/atomic.h say repeatedly
> that the "useful range of an atomic_t is only 24 bits".
>   

Old sparcs had no way to implement proper atomics, so they used to 
reserve a byte in the atomic_t as a spinlock.  But I don't think that's 
true any more; they have a separate lock hash, and atomics in the kernel 
are at least 32-bits.

> First, are the comments true in the xen implementation?
> (I don't see any code that restricts them to anything less
> than "int".)  If they ARE limited to 24 bits, what's the
> recommended approach for larger atomic operations... or
> do I just need to always use a lock?
>   

I don't think the limitation ever applied to Xen.

    J