bpf: Overwrite the htab element atomically #4946
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Add hlist_nulls_replace_rcu() to replace an existing element in the hash list. For the concurrent list traversal, the replace is atomic, it will find either the old element or the new element. Signed-off-by: Hou Tao <[email protected]>
Currently, the update of existing element in hash map involves two steps: 1) insert the new element at the head of the hash list 2) remove the old element It is possible that the concurrent lookup operation may fail to find either the old element or the new element if the lookup operation starts before the addition and continues after the removal. Therefore, replacing the two-step update with an atomic update. After the change, the update will be atomic in the perspective of the lookup operation: it will either find the old element or the new element. Signed-off-by: Hou Tao <[email protected]>
Add a test case to verify the atomic update of existing element in hash map. The test proceeds in three steps: 1) fill the map with keys in the range [0, 63] 2) create 8 threads to lookup these keys concurrently 3) create 2 threads to overwrite these keys concurrently Without atomic update support, the lookup operation may return -ENOENT error and the test will fail. After the atomic-update change, the lookup operation will always return 0 and the test will succeed. Signed-off-by: Hou Tao <[email protected]>
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Pull request for series with
subject: bpf: Overwrite the htab element atomically
version: 1
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=930344