[PATCH] drm/panfrost: Fix dma_resv deadlock at drm object pin time

[Adrián Larumbe]

When Panfrost must pin an object that is being prepared a dma-buf
attachment for on behalf of another driver, the core drm gem object pinning
code already takes a lock on the object's dma reservation.

However, Panfrost GEM object's pinning callback would eventually try taking
the lock on the same dma reservation when delegating pinning of the object
onto the shmem subsystem, which led to a deadlock.

This can be shown by enabling CONFIG_DEBUG_WW_MUTEX_SLOWPATH, which throws
the following recursive locking situation:

weston/3440 is trying to acquire lock:
ffff000000e235a0 (reservation_ww_class_mutex){+.+.}-{3:3}, at: drm_gem_shmem_pin+0x34/0xb8 [drm_shmem_helper]
but task is already holding lock:
ffff000000e235a0 (reservation_ww_class_mutex){+.+.}-{3:3}, at: drm_gem_pin+0x2c/0x80 [drm]

Fix it by assuming the object's reservation had already been locked by the
time we reach panfrost_gem_pin.

Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
Cc: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
Cc: Steven Price <steven.price@xxxxxxx>
Fixes: a78027847226 ("drm/gem: Acquire reservation lock in drm_gem_{pin/unpin}()")
Signed-off-by: Adrián Larumbe <adrian.larumbe@xxxxxxxxxxxxx>
---
 drivers/gpu/drm/panfrost/panfrost_gem.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.c b/drivers/gpu/drm/panfrost/panfrost_gem.c
index d47b40b82b0b..6c26652d425d 100644
--- a/drivers/gpu/drm/panfrost/panfrost_gem.c
+++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
@@ -192,7 +192,12 @@ static int panfrost_gem_pin(struct drm_gem_object *obj)
 	if (bo->is_heap)
 		return -EINVAL;
 
-	return drm_gem_shmem_pin(&bo->base);
+	/*
+	 * Pinning can only happen in response to a prime attachment request from
+	 * another driver, but that's already being handled by drm_gem_pin
+	 */
+	drm_WARN_ON(obj->dev, obj->import_attach);
+	return drm_gem_shmem_pin_locked(&bo->base);
 }
 
 static enum drm_gem_object_status panfrost_gem_status(struct drm_gem_object *obj)

base-commit: 04687bff66b8a4b22748aa7215d3baef0b318e5b
-- 
2.44.0