linux-kernel - [PATCH v19 15/30] drm/shmem-helper: Avoid lockdep warning when pages are released

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Date: Fri,  5 Jan 2024 21:46:09 +0300
From: Dmitry Osipenko <dmitry.osipenko@...labora.com>
To: David Airlie <airlied@...il.com>,
	Gerd Hoffmann <kraxel@...hat.com>,
	Gurchetan Singh <gurchetansingh@...omium.org>,
	Chia-I Wu <olvaffe@...il.com>,
	Daniel Vetter <daniel@...ll.ch>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Christian König <christian.koenig@....com>,
	Qiang Yu <yuq825@...il.com>,
	Steven Price <steven.price@....com>,
	Boris Brezillon <boris.brezillon@...labora.com>,
	Emma Anholt <emma@...olt.net>,
	Melissa Wen <mwen@...lia.com>
Cc: dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org,
	kernel@...labora.com,
	virtualization@...ts.linux-foundation.org
Subject: [PATCH v19 15/30] drm/shmem-helper: Avoid lockdep warning when pages are released

All drivers will be moved to get/put pages explicitly and then the last
put_pages() will be invoked during gem_free() time by some drivers.
We can't touch reservation lock when GEM is freed because that will cause
a spurious warning from lockdep when shrinker support will be added.
Lockdep doesn't know that fs_reclaim isn't functioning for a freed object,
and thus, can't deadlock. Release pages directly without taking reservation
lock if GEM is freed and its refcount is zero.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index f5ed64f78648..c7357110ca76 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -242,6 +242,22 @@ void drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem)
 	if (refcount_dec_not_one(&shmem->pages_use_count))
 		return;
 
+	/*
+	 * Destroying the object is a special case because acquiring
+	 * the obj lock 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 GEM is released.  Unfortunately
+	 * lockdep is not aware of this detail.  So when the refcount drops
+	 * to zero, we pretend it is already locked.
+	 */
+	if (!kref_read(&shmem->base.refcount)) {
+		if (refcount_dec_and_test(&shmem->pages_use_count))
+			drm_gem_shmem_free_pages(shmem);
+		return;
+	}
+
 	dma_resv_lock(shmem->base.resv, NULL);
 	drm_gem_shmem_put_pages_locked(shmem);
 	dma_resv_unlock(shmem->base.resv);
-- 
2.43.0