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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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".

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.

And a peek into xa implementation told me the internal lock is not 
protecting "next.

I could stick with one lock and not use the internal one if I used it on 
release path as well.

Regards,

Tvrtko
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