[PATCH v4 0/6] Move dma-buf mmap() reservation locking down to exporters

Dmitry Osipenko
2023-05-29T22:46:18Z

This patchset makes dma-buf exporters responisble for taking care of
the reservation lock. I also included patch that moves drm-shmem to use
reservation lock, to let CI test the whole set. I'm going to take all
the patches via the drm-misc tree, please give an ack.

Previous policy stated that dma-buf core takes the lock around mmap()
callback. Which meant that both importers and exporters shouldn't touch
the reservation lock in the mmap() code path. This worked well until
Intel-CI found a deadlock problem in a case of self-imported dma-buf [1].

The problem happens when userpace mmaps a self-imported dma-buf, i.e.
mmaps the dma-buf FD. DRM core treats self-imported dma-bufs as own GEMs
[2]. There is no way to differentiate a prime GEM from a normal GEM for
drm-shmem in drm_gem_shmem_mmap(), which resulted in a deadlock problem
for drm-shmem mmap() code path once it's switched to use reservation lock.

It was difficult to fix the drm-shmem problem without adjusting dma-buf
locking policy. In parctice not much changed from importers perspective
because previosly dma-buf was taking the lock in between of importers
and exporters. Now this lock is shifted down to exporters.

[1] https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_114671v2/shard-snb5/igt@prime_vgem@sync@rcs0.html
[2] https://elixir.bootlin.com/linux/v6.3-rc4/source/drivers/gpu/drm/drm_prime.c#L924

Changelog:

v4: - Added dma_resv_assert_held() to drm_gem_shmem_get_pages(), making
assert consistent with the put() variant. Suggested by Emil Velikov.

v3: - Added r-b from Hans Verkuil to the videobuf2 patch.

- The v2 fastrpc patch was already applied, not including it anymore.
Though, the cover-letter says that I'd want to apply all the patches
via the drm-misc tree to keep the proper ordering of the changes.

- Previously Intel's CI gave a flake failure to v2, want to re-test
it again.

v2: - Added ack from Christian König to the DRM patch.

- Dropped "fixes" tag from the patches, like was requested by
Christian König. The patches don't actually need a backport
and merely improve the locking policy.

- Dropped "reverts" from the patch titles to prevent them from
auto-backporting by the stable bot based on the title.

- Added r-b from Emil Velikov and placed the drm_WARN in the
drm-shmem patch like he suggested in a comment to v1.

- Corrected drm-shmem patch dma_resv_lock(obj->resv) inconsistently
used with dma_resv_unlock(shmem->base.resv). Now shmem->base.resv
variant is used for all locks/unlocks.

Dmitry Osipenko (6):
media: videobuf2: Don't assert held reservation lock for dma-buf
mmapping
dma-buf/heaps: Don't assert held reservation lock for dma-buf mmapping
udmabuf: Don't assert held reservation lock for dma-buf mmapping
drm: Don't assert held reservation lock for dma-buf mmapping
dma-buf: Change locking policy for mmap()
drm/shmem-helper: Switch to reservation lock

drivers/dma-buf/dma-buf.c | 17 +-
drivers/dma-buf/heaps/cma_heap.c | 3 -
drivers/dma-buf/heaps/system_heap.c | 3 -
drivers/dma-buf/udmabuf.c | 2 -
drivers/gpu/drm/drm_gem_shmem_helper.c | 210 ++++++++----------
drivers/gpu/drm/drm_prime.c | 2 -
drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c | 2 -
drivers/gpu/drm/lima/lima_gem.c | 8 +-
drivers/gpu/drm/omapdrm/omap_gem_dmabuf.c | 2 -
drivers/gpu/drm/panfrost/panfrost_drv.c | 7 +-
.../gpu/drm/panfrost/panfrost_gem_shrinker.c | 6 +-
drivers/gpu/drm/panfrost/panfrost_mmu.c | 19 +-
drivers/gpu/drm/tegra/gem.c | 2 -
.../common/videobuf2/videobuf2-dma-contig.c | 3 -
.../media/common/videobuf2/videobuf2-dma-sg.c | 3 -
.../common/videobuf2/videobuf2-vmalloc.c | 3 -
include/drm/drm_gem_shmem_helper.h | 14 +-
17 files changed, 119 insertions(+), 187 deletions(-)

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2.40.1