Re: [PATCH 8/8] drm/panfrost: Make sure the shrinker does not reclaim referenced BOs

[Boris Brezillon <boris.brezillon@collabora.com>]

On Mon, 2 Dec 2019 12:50:20 +0000
Robin Murphy <robin.murphy@arm.com> wrote:

> On 29/11/2019 1:59 pm, Boris Brezillon wrote:
> > Userspace might tag a BO purgeable while it's still referenced by GPU
> > jobs. We need to make sure the shrinker does not purge such BOs until
> > all jobs referencing it are finished.  
> 
> Nit: for extra robustness, perhaps it's worth using the refcount_t API 
> rather than bare atomic_t?

I considered doing that. The problem is, we start counting from 0, not
1, and the refcount API assumes counters start at 0, and should never
see a 0 -> 1 transition. I guess we could do

	if (refcount_inc_not_zero()) {
		...
	} else {
		refcount_set(1);
		...
	}

so we at least get the integer overflow/underflow protection.

Anyway, I'm reworking the gem_shmem code so we can refcount the sgt
users (I actually re-use the page_use_count counter and turn into a
refcount_t so we don't need to take the lock if it's > 0). With this
change I think I won't need the gpu_usecount.