On Mon, 12 Jan 2026 12:33:33 +0000 Steven Price <[email protected]> wrote:
> On 09/01/2026 13:08, Boris Brezillon wrote: > > This will be used to order things by reclaimability. > > > > Signed-off-by: Boris Brezillon <[email protected]> > > --- > > 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 44f05bd957e7..458d22380e96 100644 > > --- a/drivers/gpu/drm/panthor/panthor_gem.c > > +++ b/drivers/gpu/drm/panthor/panthor_gem.c > > @@ -484,6 +484,7 @@ 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) > > @@ -600,6 +601,13 @@ 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); > > + if (!refcount_inc_not_zero(&bo->cmap.mmap_count)) > > + refcount_set(&bo->cmap.mmap_count, 1); > > + dma_resv_unlock(obj->resv); > > + } > > + > > vm_flags_set(vma, VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP); > > vma->vm_page_prot = vm_get_page_prot(vma->vm_flags); > > if (should_map_wc(bo)) > > @@ -732,10 +740,42 @@ static vm_fault_t panthor_gem_fault(struct vm_fault > > *vmf) > > return blocking_page_setup(vmf, bo, page_offset, true); > > } > > > > +static void panthor_gem_vm_open(struct vm_area_struct *vma) > > +{ > > + struct panthor_gem_object *bo = to_panthor_bo(vma->vm_private_data); > > + > > + /* mmap_count must have been incremented at mmap time, so it can't be > > + * zero here. > > + */ > > + if (!drm_gem_is_imported(&bo->base)) > > + drm_WARN_ON(bo->base.dev, > > !refcount_inc_not_zero(&bo->cmap.mmap_count)); > > + > > + drm_gem_vm_open(vma); > > +} > > + > > +static void panthor_gem_vm_close(struct vm_area_struct *vma) > > +{ > > + struct panthor_gem_object *bo = to_panthor_bo(vma->vm_private_data); > > + > > + if (drm_gem_is_imported(&bo->base)) > > + goto out; > > + > > + if (refcount_dec_not_one(&bo->cmap.mmap_count)) > > + goto out; > > + > > + dma_resv_lock(bo->base.resv, NULL); > > + if (!refcount_dec_not_one(&bo->cmap.mmap_count)) > > + refcount_set(&bo->cmap.mmap_count, 0); > > + dma_resv_unlock(bo->base.resv); > > I don't think this logic is safe. Holding the resv_lock doesn't protect > against another thread doing a refcount_inc_not_zero() without holding > the lock. > > I think you can just replace the if() part with a refcount_dec() call, > the lock AFAICT is needed because the following patch wants to be sure > that !!mmap_count is stable when resv_lock is held. I wish I could, but refcount_dec() doesn't let me do the 1 -> 0 without complaining :P. > > I also feel you should invert the conditino for refcount_dec_not_one, > leading to the following which I feel is easier to read: > > static void panthor_gem_vm_close(struct vm_area_struct *vma) > { > [...] > > if (!refcount_dec_not_one(&bo->cmap.mmap_count)) { > dma_resv_lock(bo->base.resv, NULL); > refcount_dec(&bo->cmap.mmap_count); > dma_resv_unlock(bo->base.resv); > } The best I can do is: if (!refcount_dec_not_one(&bo->cmap.mmap_count)) { dma_resv_lock(bo->base.resv, NULL); if (!refcount_dec_not_one(&bo->cmap.mmap_count)) refcount_set(&bo->cmap.mmap_count, 0); dma_resv_unlock(bo->base.resv); } so we only take the lock when absolutely needed, but the 1 -> 0 transition still has to be done with "if (dec_not_one) set(0)". > > drm_gem_object_put(&bo->base); > }
