Re: [PATCH v2 7/8] drm/panthor: Track the number of mmap on a BO

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

On Mon, 2 Feb 2026 16:48:41 +0000
Steven Price <steven.price@arm.com> wrote:

> On 02/02/2026 11:36, Boris Brezillon wrote:
> > This will be used to order things by reclaimability.
> > 
> > v2:
> > - Fix refcounting
> > 
> > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> > ---
> >  drivers/gpu/drm/panthor/panthor_gem.c | 44 +++++++++++++++++++++++++--
> >  drivers/gpu/drm/panthor/panthor_gem.h |  3 ++
> >  2 files changed, 45 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/panthor/panthor_gem.c b/drivers/gpu/drm/panthor/panthor_gem.c
> > index 7e966fbe500f..26fe4be10a86 100644
> > --- a/drivers/gpu/drm/panthor/panthor_gem.c
> > +++ b/drivers/gpu/drm/panthor/panthor_gem.c
> > @@ -491,6 +491,7 @@ static void panthor_gem_print_info(struct drm_printer *p, unsigned int indent,
> >  	drm_printf_indent(p, indent, "vmap_use_count=%u\n",
> >  			  refcount_read(&bo->cmap.vaddr_use_count));
> >  	drm_printf_indent(p, indent, "vaddr=%p\n", bo->cmap.vaddr);
> > +	drm_printf_indent(p, indent, "mmap_count=%u\n", refcount_read(&bo->cmap.mmap_count));
> >  }
> >  
> >  static int panthor_gem_pin_locked(struct drm_gem_object *obj)
> > @@ -603,6 +604,12 @@ static int panthor_gem_mmap(struct drm_gem_object *obj, struct vm_area_struct *v
> >  	if (is_cow_mapping(vma->vm_flags))
> >  		return -EINVAL;
> >  
> > +	if (!refcount_inc_not_zero(&bo->cmap.mmap_count)) {
> > +		dma_resv_lock(obj->resv, NULL);
> > +		refcount_set(&bo->cmap.mmap_count, 1);  
> 
> I think you still need to recheck the refcount with the lock held.
> Otherwise two threads could race:
> 
>  Thread 1			| Thread 2
>  -------------------------------+--------------------------
>  if (!refcount_inc_not_zero())  |
> 	<pre-empted>		|
> 				| if (!refcount_inc_not_zero())
> 				| dma_resv_lock()
> 				| refcount_set(..., 1)
> 				| dma_resv_unlock()
>  dma_resv_lock()		|
>  refcount_set(..., 1)		|
>  dma_resv_unlock()		|
> 
> Which leaves a refcount missing.

Uh, right. I fixed that pattern in multiple places but somehow
overlooked this one. Will fix in v3.

Thanks!