Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it

[Matthew Wilcox <matthew@wil.cx>]

On Wed, Dec 06, 2006 at 11:05:22AM -0800, Linus Torvalds wrote:
> On Wed, 6 Dec 2006, Christoph Lameter wrote:
> >
> > I'd really appreciate a cmpxchg that is generically available for 
> > all arches. It will allow lockless implementation for various performance 
> > criticial portions of the kernel.
> 
> I suspect ARM may have been the last one without one, no?

It's just been pointed out to me that the parisc one isn't safe.

<dhowells> imagine variable X is set to 3
<dhowells> CPU A issues cmpxchg(&X, 3, 5)
<dhowells> you'd expect that to change X to 5
<dhowells> but what if CPU B assigns 6 to X between cmpxchg reading X
and it setting X?

Given parisc's paucity of atomic operations (load-and-zero-32bit and
load-and-zero-64bit), cmpxchg() is impossible to implement safely.
There has to be something we can hook to exclude another processor
modifying the variable.  I'm OK with using atomic_cmpxchg(); we have
atomic_set locked against it.

Of course, using cmpxchg() isn't really lockless.  It's just hidden
locking.