[PATCH v18 08/26] drm/shmem-helper: Add and use lockless drm_gem_shmem_get_pages()

[Dmitry Osipenko]

Add lockless drm_gem_shmem_get_pages() helper that skips taking reservation
lock if pages_use_count is non-zero, leveraging from atomicity of the
refcount_t. Make drm_gem_shmem_mmap() to utilize the new helper.

Reviewed-by: Boris Brezillon <boris.brezil...@collabora.com>
Suggested-by: Boris Brezillon <boris.brezil...@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipe...@collabora.com>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c 
b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 6e02643ed87e..41b749bedb11 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -226,6 +226,20 @@ void drm_gem_shmem_put_pages_locked(struct 
drm_gem_shmem_object *shmem)
 }
 EXPORT_SYMBOL_GPL(drm_gem_shmem_put_pages_locked);
 
+static int drm_gem_shmem_get_pages(struct drm_gem_shmem_object *shmem)
+{
+       int ret;
+
+       if (refcount_inc_not_zero(&shmem->pages_use_count))
+               return 0;
+
+       dma_resv_lock(shmem->base.resv, NULL);
+       ret = drm_gem_shmem_get_pages_locked(shmem);
+       dma_resv_unlock(shmem->base.resv);
+
+       return ret;
+}
+
 static int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem)
 {
        int ret;
@@ -609,10 +623,7 @@ int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, 
struct vm_area_struct
                return ret;
        }
 
-       dma_resv_lock(shmem->base.resv, NULL);
-       ret = drm_gem_shmem_get_pages_locked(shmem);
-       dma_resv_unlock(shmem->base.resv);
-
+       ret = drm_gem_shmem_get_pages(shmem);
        if (ret)
                return ret;
 
-- 
2.41.0