RDMA/uverbs: Atomically flush and mark closed the comp event queue

Currently a uverbs completion event queue is flushed of events in
ib_uverbs_comp_event_close() with the queue spinlock held and then
released.  Yet setting ev_queue->is_closed is not set until later in
uverbs_hot_unplug_completion_event_file().

In between the time ib_uverbs_comp_event_close() releases the lock and
uverbs_hot_unplug_completion_event_file() acquires the lock, a completion
event can arrive and be inserted into the event queue by
ib_uverbs_comp_handler().

This can cause a "double add" list_add warning or crash depending on the
kernel configuration, or a memory leak because the event is never dequeued
since the queue is already closed down.

So add setting ev_queue->is_closed = 1 to ib_uverbs_comp_event_close().

Cc: stable@vger.kernel.org
Fixes: 1e7710f3f6 ("IB/core: Change completion channel to use the reworked objects schema")
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/uverbs_main.c

@@ -440,6 +440,7 @@ static int ib_uverbs_comp_event_close(struct inode *inode, struct file *filp)
 			list_del(&entry->obj_list);
 		kfree(entry);
 	}
+	file->ev_queue.is_closed = 1;
 	spin_unlock_irq(&file->ev_queue.lock);
 
 	uverbs_close_fd(filp);