bpf: wait for running BPF programs when updating map-in-map
The map-in-map frequently serves as a mechanism for atomic snapshotting of state that a BPF program might record. The current implementation is dangerous to use in this way, however, since userspace has no way of knowing when all programs that might have retrieved the "old" value of the map may have completed. This change ensures that map update operations on map-in-map map types always wait for all references to the old map to drop before returning to userspace. Signed-off-by: NDaniel Colascione <dancol@google.com> Reviewed-by: NJoel Fernandes (Google) <joel@joelfernandes.org> Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
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