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: eb8d0f5a ("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

drivers/gpu/drm/i915/i915_debugfs.c

@@ -1280,14 +1280,11 @@ static int i915_hangcheck_info(struct seq_file *m, void *unused)
 	intel_wakeref_t wakeref;
 	enum intel_engine_id id;
 
+	seq_printf(m, "Reset flags: %lx\n", dev_priv->gpu_error.flags);
 	if (test_bit(I915_WEDGED, &dev_priv->gpu_error.flags))
-		seq_puts(m, "Wedged\n");
+		seq_puts(m, "\tWedged\n");
 	if (test_bit(I915_RESET_BACKOFF, &dev_priv->gpu_error.flags))
-		seq_puts(m, "Reset in progress: struct_mutex backoff\n");
-	if (waitqueue_active(&dev_priv->gpu_error.wait_queue))
-		seq_puts(m, "Waiter holding struct mutex\n");
-	if (waitqueue_active(&dev_priv->gpu_error.reset_queue))
-		seq_puts(m, "struct_mutex blocked for reset\n");
+		seq_puts(m, "\tDevice (global) reset in progress\n");
 
 	if (!i915_modparams.enable_hangcheck) {
 		seq_puts(m, "Hangcheck disabled\n");
@@ -3872,9 +3869,6 @@ i915_wedged_set(void *data, u64 val)
 	 * while it is writing to 'i915_wedged'
 	 */
 
-	if (i915_reset_backoff(&i915->gpu_error))
-		return -EAGAIN;
-
 	i915_handle_error(i915, val, I915_ERROR_CAPTURE,
 			  "Manually set wedged engine mask = %llx", val);
 	return 0;

drivers/gpu/drm/i915/i915_drv.h

@@ -3001,7 +3001,12 @@ i915_gem_obj_finish_shmem_access(struct drm_i915_gem_object *obj)
 	i915_gem_object_unpin_pages(obj);
 }
 
-int __must_check i915_mutex_lock_interruptible(struct drm_device *dev);
+static inline int __must_check
+i915_mutex_lock_interruptible(struct drm_device *dev)
+{
+	return mutex_lock_interruptible(&dev->struct_mutex);
+}
+
 int i915_gem_dumb_create(struct drm_file *file_priv,
 			 struct drm_device *dev,
 			 struct drm_mode_create_dumb *args);
@@ -3018,21 +3023,11 @@ int __must_check i915_gem_set_global_seqno(struct drm_device *dev, u32 seqno);
 struct i915_request *
 i915_gem_find_active_request(struct intel_engine_cs *engine);
 
-static inline bool i915_reset_backoff(struct i915_gpu_error *error)
-{
-	return unlikely(test_bit(I915_RESET_BACKOFF, &error->flags));
-}
-
 static inline bool i915_terminally_wedged(struct i915_gpu_error *error)
 {
 	return unlikely(test_bit(I915_WEDGED, &error->flags));
 }
 
-static inline bool i915_reset_backoff_or_wedged(struct i915_gpu_error *error)
-{
-	return i915_reset_backoff(error) | i915_terminally_wedged(error);
-}
-
 static inline u32 i915_reset_count(struct i915_gpu_error *error)
 {
 	return READ_ONCE(error->reset_count);
@@ -3105,7 +3100,6 @@ struct drm_i915_fence_reg *
 i915_reserve_fence(struct drm_i915_private *dev_priv);
 void i915_unreserve_fence(struct drm_i915_fence_reg *fence);
 
-void i915_gem_revoke_fences(struct drm_i915_private *dev_priv);
 void i915_gem_restore_fences(struct drm_i915_private *dev_priv);
 
 void i915_gem_detect_bit_6_swizzle(struct drm_i915_private *dev_priv);

drivers/gpu/drm/i915/i915_gem.c

@@ -98,47 +98,6 @@ static void i915_gem_info_remove_obj(struct drm_i915_private *dev_priv,
 	spin_unlock(&dev_priv->mm.object_stat_lock);
 }
 
-static int
-i915_gem_wait_for_error(struct i915_gpu_error *error)
-{
-	int ret;
-
-	might_sleep();
-
-	/*
-	 * Only wait 10 seconds for the gpu reset to complete to avoid hanging
-	 * userspace. If it takes that long something really bad is going on and
-	 * we should simply try to bail out and fail as gracefully as possible.
-	 */
-	ret = wait_event_interruptible_timeout(error->reset_queue,
-					       !i915_reset_backoff(error),
-					       I915_RESET_TIMEOUT);
-	if (ret == 0) {
-		DRM_ERROR("Timed out waiting for the gpu reset to complete\n");
-		return -EIO;
-	} else if (ret < 0) {
-		return ret;
-	} else {
-		return 0;
-	}
-}
-
-int i915_mutex_lock_interruptible(struct drm_device *dev)
-{
-	struct drm_i915_private *dev_priv = to_i915(dev);
-	int ret;
-
-	ret = i915_gem_wait_for_error(&dev_priv->gpu_error);
-	if (ret)
-		return ret;
-
-	ret = mutex_lock_interruptible(&dev->struct_mutex);
-	if (ret)
-		return ret;
-
-	return 0;
-}
-
 static u32 __i915_gem_park(struct drm_i915_private *i915)
 {
 	intel_wakeref_t wakeref;
@@ -1885,6 +1844,7 @@ vm_fault_t i915_gem_fault(struct vm_fault *vmf)
 	intel_wakeref_t wakeref;
 	struct i915_vma *vma;
 	pgoff_t page_offset;
+	int srcu;
 	int ret;
 
 	/* Sanity check that we allow writing into this object */
@@ -1924,7 +1884,6 @@ vm_fault_t i915_gem_fault(struct vm_fault *vmf)
 		goto err_unlock;
 	}
 
-
 	/* Now pin it into the GTT as needed */
 	vma = i915_gem_object_ggtt_pin(obj, NULL, 0, 0,
 				       PIN_MAPPABLE |
@@ -1962,9 +1921,15 @@ vm_fault_t i915_gem_fault(struct vm_fault *vmf)
 	if (ret)
 		goto err_unpin;
 
+	srcu = i915_reset_trylock(dev_priv);
+	if (srcu < 0) {
+		ret = srcu;
+		goto err_unpin;
+	}
+
 	ret = i915_vma_pin_fence(vma);
 	if (ret)
-		goto err_unpin;
+		goto err_reset;
 
 	/* Finally, remap it using the new GTT offset */
 	ret = remap_io_mapping(area,
@@ -1985,6 +1950,8 @@ vm_fault_t i915_gem_fault(struct vm_fault *vmf)
 
 err_fence:
 	i915_vma_unpin_fence(vma);
+err_reset:
+	i915_reset_unlock(dev_priv, srcu);
 err_unpin:
 	__i915_vma_unpin(vma);
 err_unlock:
@@ -5342,6 +5309,7 @@ int i915_gem_init_early(struct drm_i915_private *dev_priv)
 	init_waitqueue_head(&dev_priv->gpu_error.wait_queue);
 	init_waitqueue_head(&dev_priv->gpu_error.reset_queue);
 	mutex_init(&dev_priv->gpu_error.wedge_mutex);
+	init_srcu_struct(&dev_priv->gpu_error.reset_backoff_srcu);
 
 	atomic_set(&dev_priv->mm.bsd_engine_dispatch_index, 0);
 
@@ -5374,6 +5342,8 @@ void i915_gem_cleanup_early(struct drm_i915_private *dev_priv)
 	GEM_BUG_ON(atomic_read(&dev_priv->mm.free_count));
 	WARN_ON(dev_priv->mm.object_count);
 
+	cleanup_srcu_struct(&dev_priv->gpu_error.reset_backoff_srcu);
+
 	kmem_cache_destroy(dev_priv->priorities);
 	kmem_cache_destroy(dev_priv->dependencies);
 	kmem_cache_destroy(dev_priv->requests);

drivers/gpu/drm/i915/i915_gem_fence_reg.c

@@ -270,6 +270,10 @@ static int fence_update(struct drm_i915_fence_reg *fence,
 		return 0;
 	}
 
+	ret = i915_reset_trylock(fence->i915);
+	if (ret < 0)
+		goto out_rpm;
+
 	fence_write(fence, vma);
 	fence->vma = vma;
 
@@ -278,8 +282,12 @@ static int fence_update(struct drm_i915_fence_reg *fence,
 		list_move_tail(&fence->link, &fence->i915->mm.fence_list);
 	}
 
+	i915_reset_unlock(fence->i915, ret);
+	ret = 0;
+
+out_rpm:
 	intel_runtime_pm_put(fence->i915, wakeref);
-	return 0;
+	return ret;
 }
 
 /**
@@ -442,32 +450,6 @@ void i915_unreserve_fence(struct drm_i915_fence_reg *fence)
 	list_add(&fence->link, &fence->i915->mm.fence_list);
 }
 
-/**
- * i915_gem_revoke_fences - revoke fence state
- * @dev_priv: i915 device private
- *
- * Removes all GTT mmappings via the fence registers. This forces any user
- * of the fence to reacquire that fence before continuing with their access.
- * One use is during GPU reset where the fence register is lost and we need to
- * revoke concurrent userspace access via GTT mmaps until the hardware has been
- * reset and the fence registers have been restored.
- */
-void i915_gem_revoke_fences(struct drm_i915_private *dev_priv)
-{
-	int i;
-
-	lockdep_assert_held(&dev_priv->drm.struct_mutex);
-
-	for (i = 0; i < dev_priv->num_fence_regs; i++) {
-		struct drm_i915_fence_reg *fence = &dev_priv->fence_regs[i];
-
-		GEM_BUG_ON(fence->vma && fence->vma->fence != fence);
-
-		if (fence->vma)
-			i915_vma_revoke_mmap(fence->vma);
-	}
-}
-
 /**
  * i915_gem_restore_fences - restore fence state
  * @dev_priv: i915 device private

drivers/gpu/drm/i915/i915_gpu_error.h

@@ -204,39 +204,13 @@ struct i915_gpu_error {
 
 	atomic_t pending_fb_pin;
 
-	/**
-	 * State variable controlling the reset flow and count
-	 *
-	 * This is a counter which gets incremented when reset is triggered,
-	 *
-	 * Before the reset commences, the I915_RESET_BACKOFF bit is set
-	 * meaning that any waiters holding onto the struct_mutex should
-	 * relinquish the lock immediately in order for the reset to start.
-	 *
-	 * If reset is not completed successfully, the I915_WEDGE bit is
-	 * set meaning that hardware is terminally sour and there is no
-	 * recovery. All waiters on the reset_queue will be woken when
-	 * that happens.
-	 *
-	 * This counter is used by the wait_seqno code to notice that reset
-	 * event happened and it needs to restart the entire ioctl (since most
-	 * likely the seqno it waited for won't ever signal anytime soon).
-	 *
-	 * This is important for lock-free wait paths, where no contended lock
-	 * naturally enforces the correct ordering between the bail-out of the
-	 * waiter and the gpu reset work code.
-	 */
-	unsigned long reset_count;
-
 	/**
 	 * flags: Control various stages of the GPU reset
 	 *
-	 * #I915_RESET_BACKOFF - When we start a reset, we want to stop any
-	 * other users acquiring the struct_mutex. To do this we set the
-	 * #I915_RESET_BACKOFF bit in the error flags when we detect a reset
-	 * and then check for that bit before acquiring the struct_mutex (in
-	 * i915_mutex_lock_interruptible()?). I915_RESET_BACKOFF serves a
-	 * secondary role in preventing two concurrent global reset attempts.
+	 * #I915_RESET_BACKOFF - When we start a global reset, we need to
+	 * serialise with any other users attempting to do the same, and
+	 * any global resources that may be clobber by the reset (such as
+	 * FENCE registers).
 	 *
 	 * #I915_RESET_ENGINE[num_engines] - Since the driver doesn't need to
 	 * acquire the struct_mutex to reset an engine, we need an explicit
@@ -255,6 +229,9 @@ struct i915_gpu_error {
 #define I915_RESET_ENGINE	2
 #define I915_WEDGED		(BITS_PER_LONG - 1)
 
+	/** Number of times the device has been reset (global) */
+	u32 reset_count;
+
 	/** Number of times an engine has been reset */
 	u32 reset_engine_count[I915_NUM_ENGINES];
 
@@ -272,6 +249,8 @@ struct i915_gpu_error {
 	 */
 	wait_queue_head_t reset_queue;
 
+	struct srcu_struct reset_backoff_srcu;
+
 	struct i915_gpu_restart *restart;
 };
 

drivers/gpu/drm/i915/i915_reset.c

@@ -639,6 +639,32 @@ static void reset_prepare_engine(struct intel_engine_cs *engine)
 	engine->reset.prepare(engine);
 }
 
+static void revoke_mmaps(struct drm_i915_private *i915)
+{
+	int i;
+
+	for (i = 0; i < i915->num_fence_regs; i++) {
+		struct drm_vma_offset_node *node;
+		struct i915_vma *vma;
+		u64 vma_offset;
+
+		vma = READ_ONCE(i915->fence_regs[i].vma);
+		if (!vma)
+			continue;
+
+		if (!i915_vma_has_userfault(vma))
+			continue;
+
+		GEM_BUG_ON(vma->fence != &i915->fence_regs[i]);
+		node = &vma->obj->base.vma_node;
+		vma_offset = vma->ggtt_view.partial.offset << PAGE_SHIFT;
+		unmap_mapping_range(i915->drm.anon_inode->i_mapping,
+				    drm_vma_node_offset_addr(node) + vma_offset,
+				    vma->size,
+				    1);
+	}
+}
+
 static void reset_prepare(struct drm_i915_private *i915)
 {
 	struct intel_engine_cs *engine;
@@ -648,6 +674,7 @@ static void reset_prepare(struct drm_i915_private *i915)
 		reset_prepare_engine(engine);
 
 	intel_uc_sanitize(i915);
+	revoke_mmaps(i915);
 }
 
 static int gt_reset(struct drm_i915_private *i915, unsigned int stalled_mask)
@@ -911,50 +938,22 @@ bool i915_gem_unset_wedged(struct drm_i915_private *i915)
 	return ret;
 }
 
-struct __i915_reset {
-	struct drm_i915_private *i915;
-	unsigned int stalled_mask;
-};
-
-static int __i915_reset__BKL(void *data)
-{
-	struct __i915_reset *arg = data;
-	int err;
-
-	err = intel_gpu_reset(arg->i915, ALL_ENGINES);
-	if (err)
-		return err;
-
-	return gt_reset(arg->i915, arg->stalled_mask);
-}
-
-#if RESET_UNDER_STOP_MACHINE
-/*
- * XXX An alternative to using stop_machine would be to park only the
- * processes that have a GGTT mmap. By remote parking the threads (SIGSTOP)
- * we should be able to prevent their memmory accesses via the lost fence
- * registers over the course of the reset without the potential recursive
- * of mutexes between the pagefault handler and reset.
- *
- * See igt/gem_mmap_gtt/hang
- */
-#define __do_reset(fn, arg) stop_machine(fn, arg, NULL)
-#else
-#define __do_reset(fn, arg) fn(arg)
-#endif
-
 static int do_reset(struct drm_i915_private *i915, unsigned int stalled_mask)
 {
-	struct __i915_reset arg = { i915, stalled_mask };
 	int err, i;
 
-	err = __do_reset(__i915_reset__BKL, &arg);
+	/* Flush everyone currently using a resource about to be clobbered */
+	synchronize_srcu(&i915->gpu_error.reset_backoff_srcu);
+
+	err = intel_gpu_reset(i915, ALL_ENGINES);
 	for (i = 0; err && i < RESET_MAX_RETRIES; i++) {
-		msleep(100);
-		err = __do_reset(__i915_reset__BKL, &arg);
+		msleep(10 * (i + 1));
+		err = intel_gpu_reset(i915, ALL_ENGINES);
 	}
+	if (err)
+		return err;
 
-	return err;
+	return gt_reset(i915, stalled_mask);
 }
 
 /**
@@ -966,8 +965,6 @@ static int do_reset(struct drm_i915_private *i915, unsigned int stalled_mask)
  * Reset the chip.  Useful if a hang is detected. Marks the device as wedged
  * on failure.
  *
- * Caller must hold the struct_mutex.
- *
  * Procedure is fairly simple:
  *   - reset the chip using the reset reg
  *   - re-init context state
@@ -1274,9 +1271,12 @@ void i915_handle_error(struct drm_i915_private *i915,
 		wait_event(i915->gpu_error.reset_queue,
 			   !test_bit(I915_RESET_BACKOFF,
 				     &i915->gpu_error.flags));
-		goto out;
+		goto out; /* piggy-back on the other reset */
 	}
 
+	/* Make sure i915_reset_trylock() sees the I915_RESET_BACKOFF */
+	synchronize_rcu_expedited();
+
 	/* Prevent any other reset-engine attempt. */
 	for_each_engine(engine, i915, tmp) {
 		while (test_and_set_bit(I915_RESET_ENGINE + engine->id,
@@ -1300,6 +1300,36 @@ void i915_handle_error(struct drm_i915_private *i915,
 	intel_runtime_pm_put(i915, wakeref);
 }
 
+int i915_reset_trylock(struct drm_i915_private *i915)
+{
+	struct i915_gpu_error *error = &i915->gpu_error;
+	int srcu;
+
+	rcu_read_lock();
+	while (test_bit(I915_RESET_BACKOFF, &error->flags)) {
+		rcu_read_unlock();
+
+		if (wait_event_interruptible(error->reset_queue,
+					     !test_bit(I915_RESET_BACKOFF,
+						       &error->flags)))
+			return -EINTR;
+
+		rcu_read_lock();
+	}
+	srcu = srcu_read_lock(&error->reset_backoff_srcu);
+	rcu_read_unlock();
+
+	return srcu;
+}
+
+void i915_reset_unlock(struct drm_i915_private *i915, int tag)
+__releases(&i915->gpu_error.reset_backoff_srcu)
+{
+	struct i915_gpu_error *error = &i915->gpu_error;
+
+	srcu_read_unlock(&error->reset_backoff_srcu, tag);
+}
+
 bool i915_reset_flush(struct drm_i915_private *i915)
 {
 	int err;

drivers/gpu/drm/i915/i915_reset.h

@@ -9,6 +9,7 @@
 
 #include <linux/compiler.h>
 #include <linux/types.h>
+#include <linux/srcu.h>
 
 struct drm_i915_private;
 struct intel_engine_cs;
@@ -32,6 +33,9 @@ int i915_reset_engine(struct intel_engine_cs *engine,
 void i915_reset_request(struct i915_request *rq, bool guilty);
 bool i915_reset_flush(struct drm_i915_private *i915);
 
+int __must_check i915_reset_trylock(struct drm_i915_private *i915);
+void i915_reset_unlock(struct drm_i915_private *i915, int tag);
+
 bool intel_has_gpu_reset(struct drm_i915_private *i915);
 bool intel_has_reset_engine(struct drm_i915_private *i915);
 

drivers/gpu/drm/i915/intel_drv.h

@@ -995,9 +995,6 @@ struct intel_crtc {
 
 	struct intel_crtc_state *config;
 
-	/* global reset count when the last flip was submitted */
-	unsigned int reset_count;
-
 	/* Access to these should be protected by dev_priv->irq_lock. */
 	bool cpu_fifo_underrun_disabled;
 	bool pch_fifo_underrun_disabled;

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 @@ static int __igt_reset_evict_vma(struct drm_i915_private *i915,
 	hang_fini(&h);
 unlock:
 	mutex_unlock(&i915->drm.struct_mutex);
-	igt_global_reset_unlock(i915);
 
 	if (i915_terminally_wedged(&i915->gpu_error))
 		return -EIO;

drivers/gpu/drm/i915/selftests/mock_gem_device.c

@@ -189,6 +189,7 @@ struct drm_i915_private *mock_gem_device(void)
 
 	init_waitqueue_head(&i915->gpu_error.wait_queue);
 	init_waitqueue_head(&i915->gpu_error.reset_queue);
+	init_srcu_struct(&i915->gpu_error.reset_backoff_srcu);
 	mutex_init(&i915->gpu_error.wedge_mutex);
 
 	i915->wq = alloc_ordered_workqueue("mock", 0);