From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx>
If userspace already dropped its own reference by closing the sw_sync
fence fd we might end up in a deadlock where
dma_fence_is_signaled_locked() will trigger the release of the fence and
thus try to hold the lock to remove the fence from the list.
dma_fence_is_signaled_locked() tries to release/free the fence and hold
the lock in the process.
We fix that by changing the order operation and clean up the list and
rb-tree first.
v2: Drop the fence get/put dance and manipulate the list first (Chris Wilson)
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx>
---
drivers/dma-buf/sw_sync.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
index ef0cc08..38cc738 100644
--- a/drivers/dma-buf/sw_sync.c
+++ b/drivers/dma-buf/sw_sync.c
@@ -213,11 +213,21 @@ static void sync_timeline_signal(struct sync_timeline *obj, unsigned int inc)
obj->value += inc;
list_for_each_entry_safe(pt, next, &obj->pt_list, link) {
- if (!dma_fence_is_signaled_locked(&pt->base))
+ if (!timeline_fence_signaled(&pt->base))
break;
list_del_init(&pt->link);
rb_erase(&pt->node, &obj->pt_tree);
+
+ /*
+ * A signal callback may release the last reference to this
+ * fence, causing it to be freed. That operation has to be
+ * last to avoid a use after free inside this loop, and must
+ * be after we remove the fence from the timeline in order to
+ * prevent deadlocking on timeline->lock inside
+ * timeline_fence_release().
+ */
+ dma_fence_signal_locked(&pt->base);
}
spin_unlock_irq(&obj->lock);
--
2.9.4
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