[lkml]   [2006]   [Dec]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it
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
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2006-12-06 20:29    [W:0.089 / U:4.368 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site