core: cgroup2 support #2902
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core: cgroup2 support #2902
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Unified hierarchy is official as of Linux v4.5 and now available through a new filesystem type, cgroup2, with its own super magic. Update mount logic accordingly.
Earlier during the development of unified hierarchy, the populated event was reported through by the dedicated "cgroup.populated" file; however, the interface was updated so that it's reported through the "populated" field of "cgroup.events" file. Update populated event handling logic accordingly.
After receiving SIGCHLD, one of the ways manager_dispatch_sigchld() maps the now zombie $PID to its unit is through manager_get_unit_by_pid_cgroup() which reads /proc/$PID/cgroup and looks up the unit associated with the cgroup path. On non-unified cgroup hierarchies, a process is immediately migrated to the root cgroup on death and the cgroup lookup would always have returned the unit associated with it, making it rather pointless but safe. On unified hierarchy, a zombie remains associated with the cgroup that it was associated with at the time of death and thus manager_get_unit_by_pid_cgroup() will look up the unit properly. However, by the time manager_dispatch_sigchld() is running, the original cgroup may have become empty and it and its associated unit might already have been removed. If the cgroup path doesn't yield a match, manager_dispatch_sigchld() keeps pruning the leaf component. This means that the function may return a slice unit for a pid and as a slice doesn't have ->sigchld_event() handler, calling invoke_sigchld_event() on it causes a segfault. This patch updates invoke_sigchld_event() so that it skips calling if the handler is not set.
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This is an alternative patch set for cgroup2 support sent by Tejun Heo to the mailing list.
I haven't yet compared it with #2271, but I'm putting it out as a pull request so we
can get the whole CI machinery going.
Don't merge yet.