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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
