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    Subject[PATCH v18 08/26] drm/shmem-helper: Add and use lockless drm_gem_shmem_get_pages()
    Date
    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@collabora.com>
    Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
    Signed-off-by: Dmitry Osipenko <dmitry.osipenko@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
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