i915/gem_vm_create: Race vm-destroy against object free

Matthew postulated that we should be able to hit a race in
__i915_vm_close() between the RCU object free and vma unbind viz

  GEM_BUG_ON(!list_empty(&vm->bound_list));

due to the effect of leaving the vma on the list if we are unable to
obtain the kref to the object. Let's try and find that race.

In practice, this does not happen because to race the object free vma
cleanup against vm close requires a leak of a ppGTT vma.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Acked-by: Matthew Auld <matthew.auld@intel.com>

1 files changed, 67 insertions, 0 deletions

diff --git a/tests/i915/gem_vm_create.c b/tests/i915/gem_vm_create.c
index 8843b1b3..6d93c98a 100644
--- a/tests/i915/gem_vm_create.c
+++ b/tests/i915/gem_vm_create.c
@@ -24,6 +24,7 @@
 #include "i915/gem.h"
 #include "i915/gem_vm.h"
 #include "igt.h"
+#include "igt_rand.h"
 #include "igt_dummyload.h"
 
 static int vm_create_ioctl(int i915, struct drm_i915_gem_vm_control *ctl)
@@ -386,6 +387,69 @@ static void async_destroy(int i915)
 		igt_spin_free(i915, spin[i]);
 }
 
+static void destroy_race(int i915)
+{
+	const int ncpus = sysconf(_SC_NPROCESSORS_ONLN);
+	uint32_t *vm;
+
+	/* Check we can execute a polling spinner */
+	igt_spin_free(i915, igt_spin_new(i915, .flags = IGT_SPIN_POLL_RUN));
+
+	vm = mmap(NULL, 4096, PROT_WRITE, MAP_SHARED | MAP_ANON, -1, 0);
+	igt_assert(vm != MAP_FAILED);
+
+	for (int child = 0; child < ncpus; child++)
+		vm[child] = gem_vm_create(i915);
+
+	igt_fork(child, ncpus) {
+		uint32_t ctx = gem_context_create(i915);
+		igt_spin_t *spin;
+
+		spin = __igt_spin_new(i915, ctx, .flags = IGT_SPIN_POLL_RUN);
+		while (!READ_ONCE(vm[ncpus])) {
+			struct drm_i915_gem_context_param arg = {
+				.ctx_id = ctx,
+				.param = I915_CONTEXT_PARAM_VM,
+				.value = READ_ONCE(vm[child]),
+			};
+			igt_spin_t *nxt;
+
+			if (__gem_context_set_param(i915, &arg))
+				continue;
+
+			nxt = __igt_spin_new(i915, ctx,
+					     .flags = IGT_SPIN_POLL_RUN);
+
+			igt_spin_end(spin);
+			gem_sync(i915, spin->handle);
+			igt_spin_free(i915, spin);
+
+			usleep(1000 + hars_petruska_f54_1_random_unsafe() % 2000);
+
+			spin = nxt;
+		}
+
+		igt_spin_free(i915, spin);
+		gem_context_destroy(i915, ctx);
+	}
+
+	igt_until_timeout(5) {
+		for (int child = 0; child < ncpus; child++) {
+			gem_vm_destroy(i915, vm[child]);
+			vm[child] = gem_vm_create(i915);
+		}
+		usleep(1000 + hars_petruska_f54_1_random_unsafe() % 2000);
+	}
+
+	vm[ncpus] = 1;
+	igt_waitchildren();
+
+	for (int child = 0; child < ncpus; child++)
+		gem_vm_destroy(i915, vm[child]);
+
+	munmap(vm, 4096);
+}
+
 igt_main
 {
 	int i915 = -1;
@@ -418,6 +482,9 @@ igt_main
 
 		igt_subtest("async-destroy")
 			async_destroy(i915);
+
+		igt_subtest("destroy-race")
+			destroy_race(i915);
 	}
 
 	igt_fixture {