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cgroup: handle ENODEV on cg_read_pid() gracefully

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The recently added test case TEST-07-PID1.subgroup-kill.sh surfaced a
race: if we enumerate PIDs in a cgroup, and the cgroup is unlinked at
the very same time reading will result in ENODEV. We need to handle that
gracefully. Hence let's do so.

Noticed while looking at:

https://github.com/systemd/systemd/actions/runs/16143084441/job/45554929264?pr=38120
This commit is contained in:
Lennart Poettering
2025-07-09 14:28:28 +02:00
committed by Mike Yuan
parent da412f3be2
commit 726183627b
4 changed files with 22 additions and 4 deletions
Download Patch File Download Diff File Expand all files Collapse all files

10
src/basic/cgroup-util.c
View File

@@ -142,6 +142,9 @@ int cg_read_pid(FILE *f, pid_t *ret, CGroupFlags flags) {
assert(f);
assert(ret);
/* NB: The kernel returns ENODEV if we tried to read from cgroup.procs of a cgroup that has been
* removed already. Callers should handle that! */
for (;;) {
errno = 0;
if (fscanf(f, "%lu", &ul) != 1) {
@@ -300,6 +303,13 @@ int cg_kill(
_cleanup_(pidref_done) PidRef pidref = PIDREF_NULL;
r = cg_read_pidref(f, &pidref, flags);
if (r == -ENODEV) {
/* reading from cgroup.pids will result in ENODEV if the cgroup is
* concurrently removed. Just leave in that case, because a removed cgroup
* contains no processes anymore. */
done = true;
break;
}
if (r < 0)
return RET_GATHER(ret, log_debug_errno(r, "Failed to read pidref from cgroup '%s': %m", path));
if (r == 0)

7
src/core/dbus-unit.c
View File

@@ -1351,10 +1351,13 @@ static int append_cgroup(sd_bus_message *reply, const char *p, Set *pids) {
/* libvirt / qemu uses threaded mode and cgroup.procs cannot be read at the lower levels.
* From https://docs.kernel.org/admin-guide/cgroup-v2.html#threads, “cgroup.procs” in a
* threaded domain cgroup contains the PIDs of all processes in the subtree and is not
* readable in the subtree proper. */
* readable in the subtree proper.
*
* We'll see ENODEV when trying to enumerate processes and the cgroup is removed at the same
* time. Handle this gracefully. */
r = cg_read_pidref(f, &pidref, /* flags = */ 0);
if (IN_SET(r, 0, -EOPNOTSUPP))
if (IN_SET(r, 0, -EOPNOTSUPP, -ENODEV))
break;
if (r < 0)
return r;

2
src/shared/cgroup-setup.c
View File

@@ -367,6 +367,8 @@ int cg_migrate(
if (r < 0)
return RET_GATHER(ret, r);
}
if (r == -ENODEV)
continue;
if (r < 0)
return RET_GATHER(ret, r);
} while (!done);

7
src/shared/cgroup-show.c
View File

@@ -107,9 +107,12 @@ static int show_cgroup_one_by_path(
/* libvirt / qemu uses threaded mode and cgroup.procs cannot be read at the lower levels.
* From https://docs.kernel.org/admin-guide/cgroup-v2.html#threads,
* “cgroup.procs” in a threaded domain cgroup contains the PIDs of all processes in
* the subtree and is not readable in the subtree proper. */
* the subtree and is not readable in the subtree proper.
*
* ENODEV is generated when we enumerate processes from a cgroup and the cgroup is removed
* concurrently. */
r = cg_read_pid(f, &pid, /* flags = */ 0);
if (IN_SET(r, 0, -EOPNOTSUPP))
if (IN_SET(r, 0, -EOPNOTSUPP, -ENODEV))
break;
if (r < 0)
return r;