Re: [PATCH v4 2/6] dma-buf/heaps: Don't assert held reservation lock for dma-buf mmapping

["T.J. Mercier" <tjmercier@google.com>]

On Mon, May 29, 2023 at 3:46 PM Dmitry Osipenko
<dmitry.osipenko@collabora.com> wrote:
>
> Don't assert held dma-buf reservation lock on memory mapping of exported
> buffer.
>
> We're going to change dma-buf mmap() locking policy such that exporters
> will have to handle the lock. The previous locking policy caused deadlock
> problem for DRM drivers in a case of self-imported dma-bufs once these
> drivers are moved to use reservation lock universally. The problem
> solved by moving the lock down to exporters. This patch prepares dma-buf
> heaps for the locking policy update.
>
Hi Dmitry,

I see that in patch 6 of this series calls to
dma_resv_lock/dma_resv_unlock have been added to the
drm_gem_shmem_helper functions and some exporters. But I'm curious why
no dma_resv_lock/dma_resv_unlock calls were added to these two dma-buf
heap exporters for mmap?

Thanks,
T.J.

> Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> ---
>  drivers/dma-buf/heaps/cma_heap.c    | 3 ---
>  drivers/dma-buf/heaps/system_heap.c | 3 ---
>  2 files changed, 6 deletions(-)
>
> diff --git a/drivers/dma-buf/heaps/cma_heap.c b/drivers/dma-buf/heaps/cma_heap.c
> index 1131fb943992..28fb04eccdd0 100644
> --- a/drivers/dma-buf/heaps/cma_heap.c
> +++ b/drivers/dma-buf/heaps/cma_heap.c
> @@ -13,7 +13,6 @@
>  #include <linux/dma-buf.h>
>  #include <linux/dma-heap.h>
>  #include <linux/dma-map-ops.h>
> -#include <linux/dma-resv.h>
>  #include <linux/err.h>
>  #include <linux/highmem.h>
>  #include <linux/io.h>
> @@ -183,8 +182,6 @@ static int cma_heap_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
>  {
>         struct cma_heap_buffer *buffer = dmabuf->priv;
>
> -       dma_resv_assert_held(dmabuf->resv);
> -
>         if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
>                 return -EINVAL;
>
> diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c
> index e8bd10e60998..fcf836ba9c1f 100644
> --- a/drivers/dma-buf/heaps/system_heap.c
> +++ b/drivers/dma-buf/heaps/system_heap.c
> @@ -13,7 +13,6 @@
>  #include <linux/dma-buf.h>
>  #include <linux/dma-mapping.h>
>  #include <linux/dma-heap.h>
> -#include <linux/dma-resv.h>
>  #include <linux/err.h>
>  #include <linux/highmem.h>
>  #include <linux/mm.h>
> @@ -202,8 +201,6 @@ static int system_heap_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
>         struct sg_page_iter piter;
>         int ret;
>
> -       dma_resv_assert_held(dmabuf->resv);
> -
>         for_each_sgtable_page(table, &piter, vma->vm_pgoff) {
>                 struct page *page = sg_page_iter_page(&piter);
>
> --
> 2.40.1
>