[PATCH 0/4] samples/cgroup: fix issues in the event listener samples

[Song Hu]

The two event-listener samples in samples/cgroup mishandle the
lifetime of the monitored file and poll() errors in several ways.

Patch 1 fixes the poll error guard in memcg_event_listener, which
tests "errno != EAGAIN", an error poll() never reports, so an
interrupted poll (EINTR) from a signal exits the listener instead of
resuming.

Patch 2 makes memcg_event_listener exit cleanly when the monitored
cgroup is removed.  Since eea5d2bb34ba ("kernfs: Send IN_DELETE_SELF
and IN_IGNORED") kernfs delivers both events on rmdir; the sample
warns and keeps polling, so a supervisor still counts it as alive
while the monitored cgroup is gone.  Verified against a real
container lifecycle: the unpatched listener stays alive across
docker rm -f, the patched one exits.

Patch 3 arms the inotify watch before reading the counter baseline in
memcg_event_listener, closing the window in which events were lost.

Patch 4 stops cgroup_event_listener from letting dirname(3) modify
argv[1] in place, which truncated the path reported in every event
line and made the two-listener setup taught by
Documentation/admin-guide/cgroup-v1/memory.rst produce
indistinguishable output.

Patches 1 and 2 were verified against a real container lifecycle
(docker rm -f) in a QEMU/KVM virtual machine; patch 3 runs on every
listener start; patch 4 is compile-tested only.

Song Hu (4):
  samples/cgroup: fix poll error handling in memcg_event_listener
  samples/cgroup: exit cleanly when the monitored memcg is removed
  samples/cgroup: arm the inotify watch before reading the baseline
  samples/cgroup: don't let dirname() clobber the monitored path

 samples/cgroup/cgroup_event_listener.c | 18 +++++++---
 samples/cgroup/memcg_event_listener.c  | 50 +++++++++++++++++++-------
 2 files changed, 51 insertions(+), 17 deletions(-)

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2.53.0