[PATCH 0/5] dma-fence, i915: Stop allowing SLAB_TYPESAFE_BY_RCU for dma_fence

[Jason Ekstrand]

Ever since 0eafec6d3244 ("drm/i915: Enable lockless lookup of request
tracking via RCU"), the i915 driver has used SLAB_TYPESAFE_BY_RCU (it
was called SLAB_DESTROY_BY_RCU at the time) in order to allow RCU on
i915_request.  As nifty as SLAB_TYPESAFE_BY_RCU may be, it comes with
some serious disclaimers.  In particular, objects can get recycled while
RCU readers are still in-flight.  This can be ok if everyone who touches
these objects knows about the disclaimers and is careful.  However,
because we've chosen to use SLAB_TYPESAFE_BY_RCU for i915_request and
because i915_request contains a dma_fence, we've leaked
SLAB_TYPESAFE_BY_RCU and its whole pile of disclaimers to every driver
in the kernel which may consume a dma_fence.

We've tried to keep it somewhat contained by doing most of the hard work
to prevent access of recycled objects via dma_fence_get_rcu_safe().
However, a quick grep of kernel sources says that, of the 30 instances
of dma_fence_get_rcu*, only 11 of them use dma_fence_get_rcu_safe().
It's likely there bear traps in DRM and related subsystems just waiting
for someone to accidentally step in them.

This patch series stops us using SLAB_TYPESAFE_BY_RCU for i915_request
and, instead, does an RCU-safe slab free via rcu_call().  This should
let us keep most of the perf benefits of slab allocation while avoiding
the bear traps inherent in SLAB_TYPESAFE_BY_RCU.  It then removes support
for SLAB_TYPESAFE_BY_RCU from dma_fence entirely.

Note: The last patch is labled DONOTMERGE.  This was at Daniel Vetter's
request as we may want to let this bake for a couple releases before we
rip out dma_fence_get_rcu_safe entirely.

Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
Cc: Jon Bloomfield <jon.bloomfield at intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Christian König <christian.koenig at amd.com>
Cc: Dave Airlie <airlied at redhat.com>
Cc: Matthew Auld <matthew.auld at intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>

Jason Ekstrand (5):
  drm/i915: Move intel_engine_free_request_pool to i915_request.c
  drm/i915: Use a simpler scheme for caching i915_request
  drm/i915: Stop using SLAB_TYPESAFE_BY_RCU for i915_request
  dma-buf: Stop using SLAB_TYPESAFE_BY_RCU in selftests
  DONOTMERGE: dma-buf: Get rid of dma_fence_get_rcu_safe

 drivers/dma-buf/dma-fence-chain.c         |   8 +-
 drivers/dma-buf/dma-resv.c                |   4 +-
 drivers/dma-buf/st-dma-fence-chain.c      |  24 +---
 drivers/dma-buf/st-dma-fence.c            |  27 +---
 drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c |   4 +-
 drivers/gpu/drm/i915/gt/intel_engine_cs.c |   8 --
 drivers/gpu/drm/i915/i915_active.h        |   4 +-
 drivers/gpu/drm/i915/i915_request.c       | 147 ++++++++++++----------
 drivers/gpu/drm/i915/i915_request.h       |   2 -
 drivers/gpu/drm/i915/i915_vma.c           |   4 +-
 include/drm/drm_syncobj.h                 |   4 +-
 include/linux/dma-fence.h                 |  50 --------
 include/linux/dma-resv.h                  |   4 +-
 13 files changed, 110 insertions(+), 180 deletions(-)

-- 
2.31.1