linux-kernel - [PATCH v17 08/18] drm/shmem-helper: Add and use lockless drm_gem_shmem_get

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Date:   Fri, 15 Sep 2023 02:27:11 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     David Airlie <airlied@...il.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Christian König <christian.koenig@....com>,
        Qiang Yu <yuq825@...il.com>,
        Steven Price <steven.price@....com>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Emma Anholt <emma@...olt.net>, Melissa Wen <mwen@...lia.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org
Subject: [PATCH v17 08/18] drm/shmem-helper: Add and use lockless drm_gem_shmem_get_pages()

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.brezillon@...labora.com>
Suggested-by: Boris Brezillon <boris.brezillon@...labora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.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 e9d9fbd52b34..a5964f347dda 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -225,6 +225,20 @@ 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;
@@ -608,10 +622,7 @@ 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