linux-kernel - [PATCH] drm/msm/gem: Drop obj lock in msm_gem_free

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Message-Id: <20220613205032.2652374-1-robdclark@gmail.com>
Date:   Mon, 13 Jun 2022 13:50:32 -0700
From:   Rob Clark <robdclark@...il.com>
To:     dri-devel@...ts.freedesktop.org
Cc:     freedreno@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        Rob Clark <robdclark@...omium.org>,
        Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH] drm/msm/gem: Drop obj lock in msm_gem_free_object()

From: Rob Clark <robdclark@...omium.org>

The only reason we grabbed the lock was to satisfy a bunch of places
that WARN_ON() if called without the lock held.  But this angers lockdep
which doesn't realize no one else can be holding the lock by the time we
end up destroying the object (and sees what would otherwise be a locking
inversion between reservation_ww_class_mutex and fs_reclaim).

Closes: https://gitlab.freedesktop.org/drm/msm/-/issues/14
Signed-off-by: Rob Clark <robdclark@...omium.org>
---
 drivers/gpu/drm/msm/msm_gem.c |  8 --------
 drivers/gpu/drm/msm/msm_gem.h | 14 +++++++++++++-
 2 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
index 3ef7ada59392..ccc7e6d8cc30 100644
--- a/drivers/gpu/drm/msm/msm_gem.c
+++ b/drivers/gpu/drm/msm/msm_gem.c
@@ -1020,8 +1020,6 @@ static void msm_gem_free_object(struct drm_gem_object *obj)
 	list_del(&msm_obj->mm_list);
 	mutex_unlock(&priv->mm_lock);
 
-	msm_gem_lock(obj);
-
 	/* object should not be on active list: */
 	GEM_WARN_ON(is_active(msm_obj));
 
@@ -1037,17 +1035,11 @@ static void msm_gem_free_object(struct drm_gem_object *obj)
 
 		put_iova_vmas(obj);
 
-		/* dma_buf_detach() grabs resv lock, so we need to unlock
-		 * prior to drm_prime_gem_destroy
-		 */
-		msm_gem_unlock(obj);
-
 		drm_prime_gem_destroy(obj, msm_obj->sgt);
 	} else {
 		msm_gem_vunmap(obj);
 		put_pages(obj);
 		put_iova_vmas(obj);
-		msm_gem_unlock(obj);
 	}
 
 	drm_gem_object_release(obj);
diff --git a/drivers/gpu/drm/msm/msm_gem.h b/drivers/gpu/drm/msm/msm_gem.h
index d608339c1643..432032ad4aed 100644
--- a/drivers/gpu/drm/msm/msm_gem.h
+++ b/drivers/gpu/drm/msm/msm_gem.h
@@ -229,7 +229,19 @@ msm_gem_unlock(struct drm_gem_object *obj)
 static inline bool
 msm_gem_is_locked(struct drm_gem_object *obj)
 {
-	return dma_resv_is_locked(obj->resv);
+	/*
+	 * Destroying the object is a special case.. msm_gem_free_object()
+	 * calls many things that WARN_ON if the obj lock is not held.  But
+	 * acquiring the obj lock in msm_gem_free_object() can cause a
+	 * locking order inversion between reservation_ww_class_mutex and
+	 * fs_reclaim.
+	 *
+	 * This deadlock is not actually possible, because no one should
+	 * be already holding the lock when msm_gem_free_object() is called.
+	 * Unfortunately lockdep is not aware of this detail.  So when the
+	 * refcount drops to zero, we pretend it is already locked.
+	 */
+	return dma_resv_is_locked(obj->resv) || (kref_read(&obj->refcount) == 0);
 }
 
 static inline bool is_active(struct msm_gem_object *msm_obj)
-- 
2.36.1