[parisc-linux] Re: + parisc-add-ptr-compatpatch.patch added to -mm tree

[Kyle McMartin]

[Sorry about how long it has taken to get to this... I bounced it to
 parisc-linux too, so hopefully someone else can comment as well.]

On Wed, Feb 22, 2006 at 10:13:56AM +0100, Ingo Molnar wrote:
> There's only one complication i can imagine on PARISC: truly atomic 
> futex_atomic_cmpxchg_inuser() is not possible in any sane way because 
> any spinlock based cmpxchg exposes itself to userspace locking up the 
> kernel - no good. [We could in theory do something about it by imposing 
> some sort of deadline on the maximum time the spinning-on-userspace-lock 
> can take - but i dont think it's worth the trouble.]
>

Due to a complete lack of useful atomic operations on parisc, the
way I envisioned implementing the routines was serializing all futex
ops on a kernel spinlock. Since it's a userspace address, we couldn't
use an atomic hash unless we found the physical address behind it, so 
just one spinlock would do... Of course, I'm probably missing something 
critical here, though.

Cheers,
	Kyle M.