Re: [patch 1/4] net: percpufy frequently used vars -- add percpu_counter_mod_bh

[Andrew Morton <akpm@osdl.org>]

Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
>
> On Wed, Mar 08, 2006 at 03:43:21PM -0800, Andrew Morton wrote:
> > Benjamin LaHaise <bcrl@kvack.org> wrote:
> > >
> > > I think it may make more sense to simply convert local_t into a long, given 
> > > that most of the users will be things like stats counters.
> > > 
> > 
> > Yes, I agree that making local_t signed would be better.  It's consistent
> > with atomic_t, atomic64_t and atomic_long_t and it's a bit more flexible.
> > 
> > Perhaps.  A lot of applications would just be upcounters for statistics,
> > where unsigned is desired.  But I think the consistency argument wins out.
> 
> It already is... for most of the arches except x86_64.

x86 uses unsigned long.

> And on -mm, the asm-generic version uses atomic_long_t for local_t (signed
> long) which seems right.

No, it uses unsigned long.  The only place where signedness matters is
local_read(), and there it is typecast to ulong.

> Although, I wonder why we use:
> 
> #define local_read(l) ((unsigned long)atomic_long_read(&(l)->a))
> 
> It would return a huge value if the local counter was even -1 no?

It's casting a signed long to an unsigned long.  That does the right thing.
Yes, it'll convert -1 to 0xffffffff[ffffffff].