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drm/gem: completely close gem_open vs. gem_close races
The gem flink name holds a reference onto the object itself, and this self-reference would prevent an flink'ed object from every being freed. To break that loop we remove the flink name when the last userspace handle disappears, i.e. when obj->handle_count reaches 0. Now in gem_open we drop the dev->object_name_lock between the flink name lookup and actually adding the handle. This means a concurrent gem_close of the last handle could result in the flink name getting reaped right inbetween, i.e. Thread 1 Thread 2 gem_open gem_close flink -> obj lookup handle_count drops to 0 remove flink name create_handle handle_count++ If someone now flinks this object again, we'll get a new flink name. We can close this race by removing the lock dropping and making the entire lookup+handle_create sequence atomic. Unfortunately to still be able to share the handle_create logic this requires a handle_create_tail function which drops the lock - we can't hold the object_name_lock while calling into a driver's ->gem_open callback. Note that for flink fixing this race isn't really important, since racing gem_open against gem_close is clearly a userspace bug. And no matter how the race ends, we won't leak any references. But with dma-buf where the userspace dma-buf fd itself is refcounted this is a valid sequence and hence we should fix it. Therefore this patch here is just a warm-up exercise (and for consistency between flink buffer sharing and dma-buf buffer sharing with self-imports). Also note that this extension of the critical section in gem_open protected by dev->object_name_lock only works because it's now a mutex: A spinlock would conflict with the potential memory allocation in idr_preload(). This is exercises by igt/gem_flink_race/flink_name. Signed-off-by:Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by: