[LTP] [PATCH] controllers/cgroup_regression_test.sh: mitigate potential mount error

[Cyril Hrubis <chrubis@suse.cz>]

Hi!
First of all sorry for the delayed response.

> Immediately `umount cgroup/` after `rmdir cgroup/0` is very likely to
> make the corresponding num_cgroups not decrease, and causes the
> following mount operation with overlapping subsys to fail.
> 
> A demo test script can be:
> mount -t cgroup -o hugetlb,pids xxx cgroup/
> mkdir cgroup/0
> rmdir cgroup/0
> umount cgroup/
> mount -t cgroup -o pids xxx cgroup/  <-- FAIL
> 
> The root cause is that `rmdir cgroup/0` is asynchronous in the kernel
> implementation, causing `umount cgroup/` to enter `cgroup_put` path,
> instead of `percpu_ref_kill` path.
> 
> There is no good kernel solution yet[1]. Therefore, we temporarily add
> `sleep` in the test script to ensure `umount cgroup/` is executed
> after `rmdir cgroup/0` is completed. Note that we only add `sleep` in
> the clean up phase of each test in the cgroup_regression_test.sh.
> No `sleep` is added in the cgroup_regression_6_1.sh and
> cgroup_regression_10_1.sh for the sake of pressure test.

There is always better solution than sprinking the code with sleeps,
here we can retry the mount instead, which would be faster and more
reliable. And we even have functions for this see
https://github.com/linux-test-project/ltp/wiki/Test-Writing-Guidelines#retry-a-function-in-limited-time

-- 
Cyril Hrubis
chrubis@suse.cz