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

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.

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(-)

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
>

Hi,

On 6/21/23 20:21, T.J. Mercier wrote:
> 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?

DMA-buf heaps are exporters, drm_gem_shmem_helper is importer. Locking
rules are different for importers and exporters.

DMA-heaps use own locking, they can be moved to resv lock in the future.

DMA-heaps don't protect internal data in theirs mmap() implementations,
nothing to protect there.

On Wed, Jun 21, 2023 at 11:16 AM Dmitry Osipenko
<dmitry.osipenko@collabora.com> wrote:
>
> Hi,
>
> On 6/21/23 20:21, T.J. Mercier wrote:
> > 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?
>
> DMA-buf heaps are exporters, drm_gem_shmem_helper is importer. Locking
> rules are different for importers and exporters.
>
> DMA-heaps use own locking, they can be moved to resv lock in the future.
>
> DMA-heaps don't protect internal data in theirs mmap() implementations,
> nothing to protect there.
>
Thanks.