From: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
To: Jason Ekstrand <jason@jlekstrand.net>,
dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org
Cc: "Daniel Vetter" <daniel.vetter@ffwll.ch>,
"Christian König" <christian.koenig@amd.com>,
"Matthew Auld" <matthew.auld@intel.com>,
"Dave Airlie" <airlied@redhat.com>
Subject: Re: [Intel-gfx] [PATCH 0/5] dma-fence, i915: Stop allowing SLAB_TYPESAFE_BY_RCU for dma_fence
Date: Thu, 10 Jun 2021 10:29:12 +0100 [thread overview]
Message-ID: <ae289388-911f-2928-d695-050863ee1007@linux.intel.com> (raw)
In-Reply-To: <20210609212959.471209-1-jason@jlekstrand.net>
On 09/06/2021 22:29, Jason Ekstrand wrote:
> 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.
I don't think the part about leaking is true...
> 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.
...because dma_fence_get_rcu_safe apears to be about whether the
*pointer* to the fence itself is rcu protected, not about the fence
object itself.
If one has a stable pointer to a fence dma_fence_get_rcu is I think
enough to deal with SLAB_TYPESAFE_BY_RCU used by i915_request (as dma
fence is a base object there). Unless you found a bug in rq field
recycling. But access to the dma fence is all tightly controlled so I
don't get what leaks.
> 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.
According to the rationale behind SLAB_TYPESAFE_BY_RCU traditional RCU
freeing can be a lot more costly so I think we need a clear
justification on why this change is being considered.
Regards,
Tvrtko
>
> 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@jlekstrand.net>
> Cc: Jon Bloomfield <jon.bloomfield@intel.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Dave Airlie <airlied@redhat.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@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(-)
>
next prev parent reply other threads:[~2021-06-10 9:29 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-09 21:29 [PATCH 0/5] dma-fence, i915: Stop allowing SLAB_TYPESAFE_BY_RCU for dma_fence Jason Ekstrand
2021-06-09 21:29 ` [PATCH 1/5] drm/i915: Move intel_engine_free_request_pool to i915_request.c Jason Ekstrand
2021-06-10 10:03 ` [Intel-gfx] " Tvrtko Ursulin
2021-06-10 13:57 ` Jason Ekstrand
2021-06-10 15:07 ` Tvrtko Ursulin
2021-06-10 16:32 ` Jason Ekstrand
2021-06-09 21:29 ` [PATCH 2/5] drm/i915: Use a simpler scheme for caching i915_request Jason Ekstrand
2021-06-10 10:08 ` [Intel-gfx] " Tvrtko Ursulin
2021-06-10 13:50 ` Jason Ekstrand
2021-06-09 21:29 ` [PATCH 3/5] drm/i915: Stop using SLAB_TYPESAFE_BY_RCU for i915_request Jason Ekstrand
2021-06-09 21:29 ` [PATCH 4/5] dma-buf: Stop using SLAB_TYPESAFE_BY_RCU in selftests Jason Ekstrand
2021-06-16 12:47 ` [Intel-gfx] " kernel test robot
2021-06-09 21:29 ` [PATCH 5/5] DONOTMERGE: dma-buf: Get rid of dma_fence_get_rcu_safe Jason Ekstrand
2021-06-10 6:51 ` Christian König
2021-06-10 13:59 ` Jason Ekstrand
2021-06-10 15:13 ` Daniel Vetter
2021-06-10 16:24 ` Jason Ekstrand
2021-06-10 16:37 ` Daniel Vetter
2021-06-10 16:52 ` Jason Ekstrand
2021-06-10 17:06 ` Daniel Vetter
2021-06-10 16:54 ` Christian König
2021-06-10 17:11 ` Daniel Vetter
2021-06-10 18:12 ` Christian König
2021-06-16 16:38 ` [Intel-gfx] " kernel test robot
2021-06-10 9:29 ` Tvrtko Ursulin [this message]
2021-06-10 9:39 ` [Intel-gfx] [PATCH 0/5] dma-fence, i915: Stop allowing SLAB_TYPESAFE_BY_RCU for dma_fence Christian König
2021-06-10 11:29 ` Daniel Vetter
2021-06-10 11:53 ` Daniel Vetter
2021-06-10 13:07 ` Tvrtko Ursulin
2021-06-10 13:35 ` Jason Ekstrand
2021-06-10 20:09 ` Jason Ekstrand
2021-06-10 20:42 ` Daniel Vetter
2021-06-11 6:55 ` Christian König
2021-06-11 7:20 ` Daniel Vetter
2021-06-11 7:42 ` Christian König
2021-06-11 9:33 ` Daniel Vetter
2021-06-11 10:03 ` Christian König
2021-06-11 15:08 ` Daniel Vetter
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