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Please see the detailed description in the patch commit message. The second patch is a comprehansive test, which tests both basic usecase of creating pid namespace and init separately, and a more specific usecase which shows how we can improve clone3(set_tid) usability after this change. This is generally useful as it makes clone3(set_tid) more universal, and work in all the cases evenly. Also is highly useful to CRIU to handle nested containers. v2: Use *_ONCE for ->child_reaper accesses atomicity, and avoid taking task_list lock for reading it. Rebase to master. This series is also available here: https://github.com/Snorch/linux/commits/allow-creating-pid-namespace-init-after-setns-v2/ Signed-off-by: Pavel Tikhomirov Pavel Tikhomirov (2): pid_namespace: allow opening pid_for_children before init was created selftests: Add tests for creating pidns init via setns kernel/exit.c | 2 +- kernel/fork.c | 2 +- kernel/pid.c | 5 +- kernel/pid_namespace.c | 9 - .../selftests/pid_namespace/.gitignore | 1 + .../testing/selftests/pid_namespace/Makefile | 2 +- .../pid_namespace/pidns_init_via_setns.c | 238 ++++++++++++++++++ 7 files changed, 245 insertions(+), 14 deletions(-) create mode 100644 tools/testing/selftests/pid_namespace/pidns_init_via_setns.c -- 2.53.0