signal/ptrace: fix cgroup2/freezer long runtimes
As reported in the thread: https://lkml.org/lkml/2019/10/1/789 , in kernels with commit 76f969e8 [cgroup: cgroup v2 freezer], we were seeing much longer runtime in strace/ptrace tests (4 minutes versus 4 seconds). The issue only manifests if CONFIG_PREEMPT is enabled, which is in all of the default configurations. As sugggested in the thread, the movement of preempt_enable_no_resched() until after the cgroup is frozen returns the behaviour to pre-5.2 runtimes. Signed-off-by:Bruce Ashfield <bruce.ashfield@gmail.com>
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