Message-Id: <20221110053133.2433412-1-mani@chromium.org>
Date:   Thu, 10 Nov 2022 16:31:33 +1100
From:   Mani Milani <mani@...omium.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>,
        Mani Milani <mani@...omium.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Christian König <christian.koenig@....com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Matthew Auld <matthew.auld@...el.com>,
        Niranjana Vishwanathapura <niranjana.vishwanathapura@...el.com>,
        Nirmoy Das <nirmoy.das@...el.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org
Subject: [PATCH] drm/i915: Fix unhandled deadlock in grab_vma()

At present, the gpu thread crashes at times when grab_vma() attempts to
acquire a gem object lock when in a deadlock state.

Problems:
I identified the following 4 issues in the current code:
1. Since grab_vma() calls i915_gem_object_trylock(), which consequently
   calls ww_mutex_trylock(), to acquire lock, it does not perform any
   -EDEADLK handling; And -EALREADY handling is also unreliable,
   according to the description of ww_mutex_trylock().
2. Since the return value of grab_vma() is a boolean showing
   success/failure, it does not provide any extra information on the
   failure reason, and therefore does not provide any mechanism to its
   caller to take any action to fix a potential deadlock.
3. Current grab_vma() implementation produces inconsistent behaviour
   depending on the refcount value, without informing the caller. If
   refcount is already zero, grab_vma() neither acquires lock nor
   increments the refcount, but still returns 'true' for success! This
   means that grab_vma() returning true (for success) does not always
   mean that the gem obj is actually safely accessible.
4. Currently, calling "i915_gem_object_lock(obj,ww)" is meant to be
   followed by a consequent "i915_gem_object_unlock(obj)" ONLY if the
   original 'ww' object pointer was NULL, or otherwise not be called and
   leave the houskeeping to "i915_gem_ww_ctx_fini(ww)". There are a few
   issues with this:
   - This is not documented anywhere in the code (that I could find),
     but only explained in an older commit message.
   - This produces an inconsistent usage of the lock/unlock functions,
     increasing the chance of mistakes and issues.
   - This is not a clean design as it requires any new code that calls
     these lock/unlock functions to know their internals, as well as the
     internals of the functions calling the new code being added.

Fix:
To fix the issues above, this patch:
1. Changes grab_vma() to call i915_gem_object_lock() instead of
   i915_gem_object_trylock(), to handle -EDEADLK and -EALREADY cases.
   This should not cause any issue since the PIN_NONBLOCK flag is
   checked beforehand in the 2 cases grab_vma() is called.
2. Changes grab_vma() to return the actual error code, instead of bool.
3. Changes grab_vma() to behave consistently when returning success, by
   both incrementing the refcount and acquiring lock at all times.
4. Changes i915_gem_object_unlock() to pair with i915_gem_object_lock()
   nicely in all cases and do the housekeeping without the need for the
   caller to do anything other than simply calling lock and unlock.
5. Ensures the gem obj->obj_link is initialized and deleted from the ww
   list such that it can be tested for emptiness using list_empty().

Signed-off-by: Mani Milani <mani@...omium.org>
---

 drivers/gpu/drm/i915/gem/i915_gem_object.c |  2 +
 drivers/gpu/drm/i915/gem/i915_gem_object.h | 10 ++++-
 drivers/gpu/drm/i915/i915_gem_evict.c      | 48 ++++++++++++----------
 drivers/gpu/drm/i915/i915_gem_ww.c         |  8 ++--
 4 files changed, 41 insertions(+), 27 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
index 369006c5317f..69d013b393fb 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
@@ -78,6 +78,8 @@ void i915_gem_object_init(struct drm_i915_gem_object *obj,
 
 	INIT_LIST_HEAD(&obj->mm.link);
 
+	INIT_LIST_HEAD(&obj->obj_link);
+
 	INIT_LIST_HEAD(&obj->lut_list);
 	spin_lock_init(&obj->lut_lock);
 
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.h b/drivers/gpu/drm/i915/gem/i915_gem_object.h
index 1723af9b0f6a..7e7a61bdf52c 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object.h
+++ b/drivers/gpu/drm/i915/gem/i915_gem_object.h
@@ -219,7 +219,7 @@ static inline bool i915_gem_object_trylock(struct drm_i915_gem_object *obj,
 		return ww_mutex_trylock(&obj->base.resv->lock, &ww->ctx);
 }
 
-static inline void i915_gem_object_unlock(struct drm_i915_gem_object *obj)
+static inline void __i915_gem_object_unlock(struct drm_i915_gem_object *obj)
 {
 	if (obj->ops->adjust_lru)
 		obj->ops->adjust_lru(obj);
@@ -227,6 +227,14 @@ static inline void i915_gem_object_unlock(struct drm_i915_gem_object *obj)
 	dma_resv_unlock(obj->base.resv);
 }
 
+static inline void i915_gem_object_unlock(struct drm_i915_gem_object *obj)
+{
+	if (list_empty(&obj->obj_link))
+		__i915_gem_object_unlock(obj);
+	else
+		i915_gem_ww_unlock_single(obj);
+}
+
 static inline void
 i915_gem_object_set_readonly(struct drm_i915_gem_object *obj)
 {
diff --git a/drivers/gpu/drm/i915/i915_gem_evict.c b/drivers/gpu/drm/i915/i915_gem_evict.c
index f025ee4fa526..3eb514b4eddc 100644
--- a/drivers/gpu/drm/i915/i915_gem_evict.c
+++ b/drivers/gpu/drm/i915/i915_gem_evict.c
@@ -55,29 +55,33 @@ static int ggtt_flush(struct intel_gt *gt)
 	return intel_gt_wait_for_idle(gt, MAX_SCHEDULE_TIMEOUT);
 }
 
-static bool grab_vma(struct i915_vma *vma, struct i915_gem_ww_ctx *ww)
+static int grab_vma(struct i915_vma *vma, struct i915_gem_ww_ctx *ww)
 {
+	int err;
+
+	/* Dead objects don't need pins */
+	if (dying_vma(vma))
+		atomic_and(~I915_VMA_PIN_MASK, &vma->flags);
+
+	err = i915_gem_object_lock(vma->obj, ww);
+
 	/*
 	 * We add the extra refcount so the object doesn't drop to zero until
-	 * after ungrab_vma(), this way trylock is always paired with unlock.
+	 * after ungrab_vma(), this way lock is always paired with unlock.
 	 */
-	if (i915_gem_object_get_rcu(vma->obj)) {
-		if (!i915_gem_object_trylock(vma->obj, ww)) {
-			i915_gem_object_put(vma->obj);
-			return false;
-		}
-	} else {
-		/* Dead objects don't need pins */
-		atomic_and(~I915_VMA_PIN_MASK, &vma->flags);
-	}
+	if (!err)
+		i915_gem_object_get(vma->obj);
 
-	return true;
+	return err;
 }
 
 static void ungrab_vma(struct i915_vma *vma)
 {
-	if (dying_vma(vma))
+	if (dying_vma(vma)) {
+		/* Dead objects don't need pins */
+		atomic_and(~I915_VMA_PIN_MASK, &vma->flags);
 		return;
+	}
 
 	i915_gem_object_unlock(vma->obj);
 	i915_gem_object_put(vma->obj);
@@ -93,10 +97,11 @@ mark_free(struct drm_mm_scan *scan,
 	if (i915_vma_is_pinned(vma))
 		return false;
 
-	if (!grab_vma(vma, ww))
+	if (grab_vma(vma, ww))
 		return false;
 
 	list_add(&vma->evict_link, unwind);
+
 	return drm_mm_scan_add_block(scan, &vma->node);
 }
 
@@ -284,10 +289,12 @@ i915_gem_evict_something(struct i915_address_space *vm,
 		vma = container_of(node, struct i915_vma, node);
 
 		/* If we find any non-objects (!vma), we cannot evict them */
-		if (vma->node.color != I915_COLOR_UNEVICTABLE &&
-		    grab_vma(vma, ww)) {
-			ret = __i915_vma_unbind(vma);
-			ungrab_vma(vma);
+		if (vma->node.color != I915_COLOR_UNEVICTABLE) {
+			ret = grab_vma(vma, ww);
+			if (!ret) {
+				ret = __i915_vma_unbind(vma);
+				ungrab_vma(vma);
+			}
 		} else {
 			ret = -ENOSPC;
 		}
@@ -382,10 +389,9 @@ int i915_gem_evict_for_node(struct i915_address_space *vm,
 			break;
 		}
 
-		if (!grab_vma(vma, ww)) {
-			ret = -ENOSPC;
+		ret = grab_vma(vma, ww);
+		if (ret)
 			break;
-		}
 
 		/*
 		 * Never show fear in the face of dragons!
diff --git a/drivers/gpu/drm/i915/i915_gem_ww.c b/drivers/gpu/drm/i915/i915_gem_ww.c
index 3f6ff139478e..937b279f50fc 100644
--- a/drivers/gpu/drm/i915/i915_gem_ww.c
+++ b/drivers/gpu/drm/i915/i915_gem_ww.c
@@ -19,16 +19,14 @@ static void i915_gem_ww_ctx_unlock_all(struct i915_gem_ww_ctx *ww)
 	struct drm_i915_gem_object *obj;
 
 	while ((obj = list_first_entry_or_null(&ww->obj_list, struct drm_i915_gem_object, obj_link))) {
-		list_del(&obj->obj_link);
-		i915_gem_object_unlock(obj);
-		i915_gem_object_put(obj);
+		i915_gem_ww_unlock_single(obj);
 	}
 }
 
 void i915_gem_ww_unlock_single(struct drm_i915_gem_object *obj)
 {
-	list_del(&obj->obj_link);
-	i915_gem_object_unlock(obj);
+	list_del_init(&obj->obj_link);
+	__i915_gem_object_unlock(obj);
 	i915_gem_object_put(obj);
 }
 
-- 
2.38.1.431.g37b22c650d-goog

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