i915/gem_close_race: Mix in a contexts and a small delay to closure

Keep the old handles in a small ring so that we build up a small amount
of pressure for i915_gem_close_object() and throw in a few concurrent
contexts so we have to process an obj->lut_list containing more than one
element. And to make sure the list is truly long enough to schedule,
start leaking the contexts.

Note that the only correctness check is that the selfcopy doesn't
explode; the challenge would be to prove that the old handles are no
longer accessible via the execbuf lut. However, this is sufficient to
make sure we at least hit the interruptible spinlock used by
close-objects.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michael J. Ruhl <michael.j.ruhl@intel.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
27 jobs for master in 13 minutes and 33 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build Containers
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  Build
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build:tests-debian-autotools

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build:tests-debian-meson

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build:tests-debian-minimal

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build:tests-fedora

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build:tests-fedora-clang

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build:tests-fedora-no-libunwind

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build:tests-fedora-oldest-meson

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  Test
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test:list-undocumented-tests

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test:ninja-test

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test:ninja-test-arm64

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test:ninja-test-minimal

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test:test-list-diff

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test:verify-blacklists

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  Containers
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containers:igt

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  Deploy
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allowed to fail manual
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