Re: [PATCH 3/4] cxl: Introduce 'struct cxl_dpa_partition' and 'struct cxl_range_info'

[Dan Williams <dan.j.williams@intel.com>]

Alejandro Lucero Palau wrote:
[..]
> > +/* if this fails the caller must destroy @cxlds, there is no recovery */
> > +int cxl_dpa_setup(struct cxl_dev_state *cxlds, const struct cxl_dpa_info *info)
> > +{
> > +	struct device *dev = cxlds->dev;
> > +
> > +	guard(rwsem_write)(&cxl_dpa_rwsem);
> > +
> 
> 
> This explains to me what you meant about locking when setting the 
> resources for Type2.
> 
> 
> However, I think this is no necessary because there is no user space, or 
> that is my idea, involved when creating CXL regions for a Type2. It is 
> all up to the accel driver to do so, therefore no locking needed because 
> none is going to traverse the child resource list while 
> initialising/updating it.

Yes, no locking is needed, and that was the status quo for cxl_pci since
it was simple to audit the single user. Going forward, with multiple
users, and the fact that cxl_dev_state is not strictly private to the
cxl_core, some safety is reasonable.

> It does not harm to have it for current Type2 case, and always a good 
> idea to have it for potential future cases.

My main motivation is to help protect against someone thinking that
calling cxl_dpa_setup() twice is a workable model.