Re: xa_cmpxchg and XA_ZERO_ENTRY?

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Oct 30, 2024 at 04:51:40PM +0000, Matthew Wilcox wrote:
> On Wed, Oct 30, 2024 at 10:15:13AM -0300, Jason Gunthorpe wrote:
> > Hi Matthew,
> > 
> > Nicolin pointed this out and I was wondering what is the right thing.
> > 
> > For instance this:
> > 
> > 	xa_init(&xa);
> > 	ret = xa_reserve(&xa, 1, GFP_KERNEL);
> > 	printk("xa_reserve() = %d\n", ret);
> > 	old = xa_cmpxchg(&xa, 1, NULL, &xa, GFP_KERNEL);
> 
> You're really not supposed to be doing xa_cmpxchg() here.  Just use
> xa_store().  That's the intended way to use these APIs.

xa_store() also looses the XA_ZERO_ENTRY, it doesn't help to write an
assertion that the index was reserved.

> > The general purpose of code like the above is to just validate that
> > the xa has not been corrupted, that the index we are storing to has
> > been reserved. Maybe we can't sleep or something.
> 
> Thr intent is to provide you with an array abstraction.  You don't
> cmpxchg() pointers into an array, do you?  Almost everybody just does
> array[i] = p.

Sort of, what is desired here is test and store, not cmpxchg. You
would do that in normal arrays:

 if (WARN_ON(array[i] == NULL)
     return;
 array[i] = foo;

In xarray it would be nice to do both those under a single walk and
lock.

Jason