[PATCH v2 01/15] drm/shmem-helper: Make pages_use_count an atomic_t
Steven Price
steven.price at arm.com
Fri Aug 11 13:08:24 UTC 2023
On 09/08/2023 17:53, Boris Brezillon wrote:
> This way we can grab a pages ref without acquiring the resv lock when
> pages_use_count > 0. Need to implement asynchronous map using the
NIT: s/Need/This is needed/
> drm_gpuva_mgr when the map/unmap operation triggers a mapping split,
> requiring the new left/right regions to grab an additional page ref
> to guarantee that the pages stay pinned when the middle section is
> unmapped.
>
> Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>
> ---
> drivers/gpu/drm/drm_gem_shmem_helper.c | 28 +++++++++++++------------
> drivers/gpu/drm/lima/lima_gem.c | 2 +-
> drivers/gpu/drm/panfrost/panfrost_mmu.c | 2 +-
> include/drm/drm_gem_shmem_helper.h | 2 +-
> 4 files changed, 18 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index a783d2245599..ca6938ea1b82 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -155,7 +155,7 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
> if (shmem->pages)
> drm_gem_shmem_put_pages(shmem);
>
> - drm_WARN_ON(obj->dev, shmem->pages_use_count);
> + drm_WARN_ON(obj->dev, atomic_read(&shmem->pages_use_count));
>
> dma_resv_unlock(shmem->base.resv);
> }
> @@ -172,14 +172,14 @@ static int drm_gem_shmem_get_pages(struct drm_gem_shmem_object *shmem)
>
> dma_resv_assert_held(shmem->base.resv);
>
> - if (shmem->pages_use_count++ > 0)
> + if (atomic_inc_return(&shmem->pages_use_count) > 1)
> return 0;
>
> pages = drm_gem_get_pages(obj);
> if (IS_ERR(pages)) {
> drm_dbg_kms(obj->dev, "Failed to get pages (%ld)\n",
> PTR_ERR(pages));
> - shmem->pages_use_count = 0;
> + atomic_set(&shmem->pages_use_count, 0);
> return PTR_ERR(pages);
> }
>
> @@ -210,10 +210,10 @@ void drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem)
>
> dma_resv_assert_held(shmem->base.resv);
>
> - if (drm_WARN_ON_ONCE(obj->dev, !shmem->pages_use_count))
> + if (drm_WARN_ON_ONCE(obj->dev, !atomic_read(&shmem->pages_use_count)))
> return;
>
> - if (--shmem->pages_use_count > 0)
> + if (atomic_dec_return(&shmem->pages_use_count) > 0)
> return;
>
> #ifdef CONFIG_X86
> @@ -263,6 +263,10 @@ int drm_gem_shmem_pin(struct drm_gem_shmem_object *shmem)
>
> drm_WARN_ON(obj->dev, obj->import_attach);
>
> + /* If we are the first owner, we need to grab the lock. */
> + if (atomic_inc_not_zero(&shmem->pages_use_count))
> + return 0;
> +
Unless I'm misunderstanding I think this introduces a race where two
threads call drm_gem_shmem_pin() at the same time:
Thread1 | Thread 2
--------------------------------+------------------------------
drm_gem_shmem_pin() |
- pages_use_count == 0 so not |
incremented |
- lock taken |
drm_gem_shmem_pin_locked() |
drm_gem_shmem_get_pages() |
- pages_use_count incremented |
<thread descheduled> | drm_gem_shmem_pin()
| - pages_use_count == 1 so is it
| incremented and returns early
| without taking the lock
| Code tries to use shmem->pages
<thread rescheduled> | and blows up
drm_gem_get_pages() |
shmem->pages populated |
lock released |
I think you need to modify drm_gem_shmem_get_pages() to only increment
pages_use_count when shmem->pages has been populated. That also gets rid
of the atomic_set() in that function which scares me.
Steve
> ret = dma_resv_lock_interruptible(shmem->base.resv, NULL);
> if (ret)
> return ret;
> @@ -286,6 +290,10 @@ void drm_gem_shmem_unpin(struct drm_gem_shmem_object *shmem)
>
> drm_WARN_ON(obj->dev, obj->import_attach);
>
> + /* If we are the last owner, we need to grab the lock. */
> + if (atomic_add_unless(&shmem->pages_use_count, -1, 1))
> + return;
> +
> dma_resv_lock(shmem->base.resv, NULL);
> drm_gem_shmem_unpin_locked(shmem);
> dma_resv_unlock(shmem->base.resv);
> @@ -543,18 +551,12 @@ static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
>
> drm_WARN_ON(obj->dev, obj->import_attach);
>
> - dma_resv_lock(shmem->base.resv, NULL);
> -
> /*
> * We should have already pinned the pages when the buffer was first
> * mmap'd, vm_open() just grabs an additional reference for the new
> * mm the vma is getting copied into (ie. on fork()).
> */
> - if (!drm_WARN_ON_ONCE(obj->dev, !shmem->pages_use_count))
> - shmem->pages_use_count++;
> -
> - dma_resv_unlock(shmem->base.resv);
> -
> + drm_WARN_ON_ONCE(obj->dev, atomic_inc_return(&shmem->pages_use_count) == 1);
> drm_gem_vm_open(vma);
> }
>
> @@ -632,7 +634,7 @@ void drm_gem_shmem_print_info(const struct drm_gem_shmem_object *shmem,
> if (shmem->base.import_attach)
> return;
>
> - drm_printf_indent(p, indent, "pages_use_count=%u\n", shmem->pages_use_count);
> + drm_printf_indent(p, indent, "pages_use_count=%u\n", atomic_read(&shmem->pages_use_count));
> drm_printf_indent(p, indent, "vmap_use_count=%u\n", shmem->vmap_use_count);
> drm_printf_indent(p, indent, "vaddr=%p\n", shmem->vaddr);
> }
> diff --git a/drivers/gpu/drm/lima/lima_gem.c b/drivers/gpu/drm/lima/lima_gem.c
> index 4f9736e5f929..0116518b1601 100644
> --- a/drivers/gpu/drm/lima/lima_gem.c
> +++ b/drivers/gpu/drm/lima/lima_gem.c
> @@ -47,7 +47,7 @@ int lima_heap_alloc(struct lima_bo *bo, struct lima_vm *vm)
> }
>
> bo->base.pages = pages;
> - bo->base.pages_use_count = 1;
> + atomic_set(&bo->base.pages_use_count, 1);
>
> mapping_set_unevictable(mapping);
> }
> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> index c0123d09f699..f66e63bf743e 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -487,7 +487,7 @@ static int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as,
> goto err_unlock;
> }
> bo->base.pages = pages;
> - bo->base.pages_use_count = 1;
> + atomic_set(&bo->base.pages_use_count, 1);
> } else {
> pages = bo->base.pages;
> if (pages[page_offset]) {
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index bf0c31aa8fbe..0661f87d3bda 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -37,7 +37,7 @@ struct drm_gem_shmem_object {
> * Reference count on the pages table.
> * The pages are put when the count reaches zero.
> */
> - unsigned int pages_use_count;
> + atomic_t pages_use_count;
>
> /**
> * @madv: State for madvise
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