drm/i915: Stop rcu support for i915_address_space
The full audit is quite a bit of work: - i915_dpt has very simple lifetime (somehow we create a display pagetable vm per object, so its _very_ simple, there's only ever a single vma in there), and uses i915_vm_close(), which internally does a i915_vm_put(). No rcu. Aside: wtf is i915_dpt doing in the intel_display.c garbage collector as a new feature, instead of added as a separate file with some clean-ish interface. Also, i915_dpt unfortunately re-introduces some coding patterns from pre-dma_resv_lock conversion times. - i915_gem_proto_ctx is fully refcounted and no rcu, all protected by fpriv->proto_context_lock. - i915_gem_context is itself rcu protected, and that might leak to anything it points at. Before commit cf977e18 Author: Chris Wilson <chris@chris-wilson.co.uk> Date: Wed Dec 2 11:21:40 2020 +0000 drm/i915/gem: Spring clean debugfs and commit db80a129 Author: Chris Wilson <chris@chris-wilson.co.uk> Date: Mon Jan 18 11:08:54 2021 +0000 drm/i915/gem: Remove per-client stats from debugfs/i915_gem_objects we had a bunch of debugfs files that relied on rcu protecting everything, but those are gone now. The main one was removed even earlier with There doesn't seem to be anything left that's actually protecting stuff now that the ctx->vm itself is invariant. See commit ccbc1b97 Author: Jason Ekstrand <jason@jlekstrand.net> Date: Thu Jul 8 10:48:30 2021 -0500 drm/i915/gem: Don't allow changing the VM on running contexts (v4) Note that we drop the vm refcount before the final release of the gem context refcount, so this is all very dangerous even without rcu. Note that aside from later on creating new engines (a defunct feature) and debug output we're never looked at gem_ctx->vm for anything functional, hence why this is ok. Fingers crossed. Preceeding patches removed all vestiges of rcu use from gem_ctx->vm derferencing to make it clear it's really not used. The gem_ctx->rcu protection was introduced in commit a4e7ccda Author: Chris Wilson <chris@chris-wilson.co.uk> Date: Fri Oct 4 14:40:09 2019 +0100 drm/i915: Move context management under GEM The commit message is somewhat entertaining because it fails to mention this fact completely, and compensates that by an in-commit changelog entry that claims that ctx->vm is protected by ctx->mutex. Which was the case _before_ this commit, but no longer after it. - intel_context holds a full reference. Unfortunately intel_context is also rcu protected and the reference to the ->vm is dropped before the rcu barrier - only the kfree is delayed. So again we need to check whether that leaks anywhere on the intel_context->vm. RCU is only used to protect intel_context sitting on the breadcrumb lists, which don't look at the vm anywhere, so we are fine. Nothing else relies on rcu protection of intel_context and hence is fully protected by the kref refcount alone, which protects intel_context->vm in turn. The breadcrumbs rcu usage was added in commit c744d503 Author: Chris Wilson <chris@chris-wilson.co.uk> Date: Thu Nov 26 14:04:06 2020 +0000 drm/i915/gt: Split the breadcrumb spinlock between global and contexts its parent commit added the intel_context rcu protection: commit 14d1eaf0 Author: Chris Wilson <chris@chris-wilson.co.uk> Date: Thu Nov 26 14:04:05 2020 +0000 drm/i915/gt: Protect context lifetime with RCU given some credence to my claim that I've actually caught them all. - drm_i915_gem_object's shares_resv_from pointer has a full refcount to the dma_resv, which is a sub-refcount that's released after the final i915_vm_put() has been called. Safe. Aside: Maybe we should have a struct dma_resv_shared which is just dma_resv + kref as a stand-alone thing. It's a pretty useful pattern which other drivers might want to copy. For a bit more context see commit 4d8151ae Author: Thomas Hellström <thomas.hellstrom@linux.intel.com> Date: Tue Jun 1 09:46:41 2021 +0200 drm/i915: Don't free shared locks while shared - the fpriv->vm_xa was relying on rcu_read_lock for lookup, but that was updated in a prep patch too to just be a spinlock-protected lookup. - intel_gt->vm is set at driver load in intel_gt_init() and released in intel_gt_driver_release(). There seems to be some issue that in some error paths this is called twice, but otherwise no rcu to be found anywhere. This was added in the below commit, which unfortunately doesn't explain why this complication exists. commit e6ba7648 Author: Chris Wilson <chris@chris-wilson.co.uk> Date: Sat Dec 21 16:03:24 2019 +0000 drm/i915: Remove i915->kernel_context The proper fix most likely for this is to start using drmm_ at large scale, but that's also huge amounts of work. - i915_vma->vm is some real pain, because rcu is rcu protected, at least in the vma lookup in the context lookup cache in eb_lookup_vma(). This was added in commit 4ff4b44c Author: Chris Wilson <chris@chris-wilson.co.uk> Date: Fri Jun 16 15:05:16 2017 +0100 drm/i915: Store a direct lookup from object handle to vma This was changed to a radix tree from the hashtable in, but with the locking unchanged, in commit d1b48c1e Author: Chris Wilson <chris@chris-wilson.co.uk> Date: Wed Aug 16 09:52:08 2017 +0100 drm/i915: Replace execbuf vma ht with an idr In commit 93159e12 Author: Chris Wilson <chris@chris-wilson.co.uk> Date: Mon Mar 23 09:28:41 2020 +0000 drm/i915/gem: Avoid gem_context->mutex for simple vma lookup the locking was changed from dev->struct_mutex to rcu, which added the requirement to rcu protect i915_vma. Somehow this was missed in review (or I'm completely blind). Irrespective of all that the vma lookup cache rcu_read_lock grabs a full reference of the vma and the rcu doesn't leak further. So no impact on i915_address_space from that. I have not found any other rcu use for i915_vma, but given that it seems broken I also didn't bother to do a careful in-depth audit. Alltogether there's nothing left in-tree anymore which requires that a pointer deref to an i915_address_space is safe undre rcu_read_lock only. rcu protection of i915_address_space was introduced in commit b32fa811 Author: Chris Wilson <chris@chris-wilson.co.uk> Date: Thu Jun 20 19:37:05 2019 +0100 drm/i915/gtt: Defer address space cleanup to an RCU worker by mixing up a bugfixing (i915_address_space needs to be released from a worker) with enabling rcu support. The commit message also seems somewhat confused, because it talks about cleanup of WC pages requiring sleep, while the code and linked bugzilla are about a requirement to take dev->struct_mutex (which yes sleeps but it's a much more specific problem). Since final kref_put can be called from pretty much anywhere (including hardirq context through the scheduler's i915_active cleanup) we need a worker here. Hence that part must be kept. Ideally all these reclaim workers should have some kind of integration with our shrinkers, but for some of these it's rather tricky. Anyway, that's a preexisting condition in the codeebase that we wont fix in this patch here. We also remove the rcu_barrier in ggtt_cleanup_hw added in commit 60a4233a Author: Chris Wilson <chris@chris-wilson.co.uk> Date: Mon Jul 29 14:24:12 2019 +0100 drm/i915: Flush the i915_vm_release before ggtt shutdown Reviewed-by:Maarten Lankhorst <maarten.lankhorst@linux.intel.com> Signed-off-by:
Daniel Vetter <daniel.vetter@intel.com> Cc: Jon Bloomfield <jon.bloomfield@intel.com> Cc: Chris Wilson <chris@chris-wilson.co.uk> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Cc: Daniel Vetter <daniel.vetter@ffwll.ch> Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com> Cc: Matthew Auld <matthew.auld@intel.com> Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com> Cc: Dave Airlie <airlied@redhat.com> Cc: Jason Ekstrand <jason@jlekstrand.net> Link: https://patchwork.freedesktop.org/patch/msgid/20210902142057.929669-11-daniel.vetter@ffwll.ch
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