Re: [Intel-gfx] [RFC 01/12] drm/i915: Expose list of clients in sysfs

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tvrtko Ursulin (2020-03-09 23:26:34)
> 
> On 09/03/2020 21:34, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2020-03-09 18:31:18)
> >> +struct i915_drm_client *
> >> +i915_drm_client_add(struct i915_drm_clients *clients, struct task_struct *task)
> >> +{
> >> +       struct i915_drm_client *client;
> >> +       int ret;
> >> +
> >> +       client = kzalloc(sizeof(*client), GFP_KERNEL);
> >> +       if (!client)
> >> +               return ERR_PTR(-ENOMEM);
> >> +
> >> +       kref_init(&client->kref);
> >> +       client->clients = clients;
> >> +
> >> +       ret = mutex_lock_interruptible(&clients->lock);
> >> +       if (ret)
> >> +               goto err_id;
> >> +       ret = xa_alloc_cyclic(&clients->xarray, &client->id, client,
> >> +                             xa_limit_32b, &clients->next_id, GFP_KERNEL);
> > 
> > So what's next_id used for that explains having the over-arching mutex?
> 
> It's to give out client id's "cyclically" - before I apparently 
> misunderstood what xa_alloc_cyclic is supposed to do - I thought after 
> giving out id 1 it would give out 2 next, even if 1 was returned to the 
> pool in the meantime. But it doesn't, I need to track the start point 
> for the next search with "next".

Ok. A requirement of the API for the external counter.
 
> I want this to make intel_gpu_top's life easier, so it doesn't have to 
> deal with id recycling for all practical purposes.

Fair enough. I only worry about the radix nodes and sparse ids :)
 
> And a peek into xa implementation told me the internal lock is not 
> protecting "next.

See xa_alloc_cyclic(), seems to cover __xa_alloc_cycle (where *next is
manipulated) under the xa_lock.
-Chris
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