On 5/11/22 17:24, Christian König wrote:
Am 11.05.22 um 15:00 schrieb Daniel Vetter:
On Tue, May 10, 2022 at 04:39:53PM +0300, Dmitry Osipenko wrote:
[SNIP] Since vmapping implies implicit pinning, we can't use a separate lock in drm_gem_shmem_vmap() because we need to protect the drm_gem_shmem_get_pages(), which is invoked by drm_gem_shmem_vmap() to pin the pages and requires the dma_resv_lock to be locked.
Hence the problem is:
- If dma-buf importer holds the dma_resv_lock and invokes
dma_buf_vmap() -> drm_gem_shmem_vmap(), then drm_gem_shmem_vmap() shall not take the dma_resv_lock.
- Since dma-buf locking convention isn't specified, we can't assume
that dma-buf importer holds the dma_resv_lock around dma_buf_vmap().
The possible solutions are:
- Specify the dma_resv_lock convention for dma-bufs and make all
drivers to follow it.
- Make only DRM drivers to hold dma_resv_lock around dma_buf_vmap().
Other non-DRM drivers will get the lockdep warning.
- Make drm_gem_shmem_vmap() to take the dma_resv_lock and get deadlock
if dma-buf importer holds the lock.
...
Yeah this is all very annoying.
Ah, yes that topic again :)
I think we could relatively easily fix that by just defining and enforcing that the dma_resv_lock must have be taken by the caller when dma_buf_vmap() is called.
A two step approach should work:
- Move the call to dma_resv_lock() into the dma_buf_vmap() function and
remove all lock taking from the vmap callback implementations. 2. Move the call to dma_resv_lock() into the callers of dma_buf_vmap() and enforce that the function is called with the lock held.
I've doubts about the need to move out the dma_resv_lock() into the callers of dma_buf_vmap()..
I looked through all the dma_buf_vmap() users and neither of them interacts with dma_resv_lock() at all, i.e. nobody takes the lock in/outside of dma_buf_vmap(). Hence it's easy and more practical to make dma_buf_mmap/vmap() to take the dma_resv_lock by themselves.
It's unclear to me which driver may ever want to do the mapping under the dma_resv_lock. But if we will ever have such a driver that will need to map imported buffer under dma_resv_lock, then we could always add the dma_buf_vmap_locked() variant of the function. In this case the locking rule will sound like this:
"All dma-buf importers are responsible for holding the dma-reservation lock around the dmabuf->ops->mmap/vmap() calls."
It shouldn't be that hard to clean up. The last time I looked into it my main problem was that we didn't had any easy unit test for it.
Do we have any tests for dma-bufs at all? It's unclear to me what you are going to test in regards to the reservation locks, could you please clarify?