drm/i915: Revoke mmaps and prevent access to fence registers across reset

Previously, we were able to rely on the recursive properties of
struct_mutex to allow us to serialise revoking mmaps and reacquiring the
FENCE registers with them being clobbered over a global device reset.
I then proceeded to throw out the baby with the bath water in order to
pursue a struct_mutex-less reset.

Perusing LWN for alternative strategies, the dilemma on how to serialise
access to a global resource on one side was answered by
https://lwn.net/Articles/202847/ -- Sleepable RCU:

    1  int readside(void) {
    2      int idx;
    3      rcu_read_lock();
    4	   if (nomoresrcu) {
    5          rcu_read_unlock();
    6	       return -EINVAL;
    7      }
    8	   idx = srcu_read_lock(&ss);
    9	   rcu_read_unlock();
    10	   /* SRCU read-side critical section. */
    11	   srcu_read_unlock(&ss, idx);
    12	   return 0;
    13 }
    14
    15 void cleanup(void)
    16 {
    17     nomoresrcu = 1;
    18     synchronize_rcu();
    19     synchronize_srcu(&ss);
    20     cleanup_srcu_struct(&ss);
    21 }

No more worrying about stop_machine, just an uber-complex mutex,
optimised for reads, with the overhead pushed to the rare reset path.

However, we do run the risk of a deadlock as we allocate underneath the
SRCU read lock, and the allocation may require a GPU reset, causing a
dependency cycle via the in-flight requests. We resolve that by declaring
the driver wedged and cancelling all in-flight rendering.

v2: Use expedited rcu barriers to match our earlier timing
characteristics.
v3: Try to annotate locking contexts for sparse
v4: Reduce selftest lock duration to avoid a reset deadlock with fences
v5: s/srcu/reset_backoff_srcu/
v6: Remove more stale comments

Testcase: igt/gem_mmap_gtt/hang
Fixes: eb8d0f5af4ec ("drm/i915: Remove GPU reset dependence on struct_mutex")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190208153708.20023-2-chris@chris-wilson.co.uk

1 files changed, 2 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/selftests/intel_hangcheck.c b/drivers/gpu/drm/i915/selftests/intel_hangcheck.c
index 7b6f3bea9ef8..4886fac12628 100644
--- a/drivers/gpu/drm/i915/selftests/intel_hangcheck.c
+++ b/drivers/gpu/drm/i915/selftests/intel_hangcheck.c
@@ -1039,8 +1039,6 @@ static int __igt_reset_evict_vma(struct drm_i915_private *i915,
 
 	/* Check that we can recover an unbind stuck on a hanging request */
 
-	igt_global_reset_lock(i915);
-
 	mutex_lock(&i915->drm.struct_mutex);
 	err = hang_init(&h, i915);
 	if (err)
@@ -1138,7 +1136,9 @@ static int __igt_reset_evict_vma(struct drm_i915_private *i915,
 	}
 
 out_reset:
+	igt_global_reset_lock(i915);
 	fake_hangcheck(rq->i915, intel_engine_flag(rq->engine));
+	igt_global_reset_unlock(i915);
 
 	if (tsk) {
 		struct igt_wedge_me w;
@@ -1159,7 +1159,6 @@ fini:
 	hang_fini(&h);
 unlock:
 	mutex_unlock(&i915->drm.struct_mutex);
-	igt_global_reset_unlock(i915);
 
 	if (i915_terminally_wedged(&i915->gpu_error))
 		return -EIO;