[PATCH] cgroup/cpuset: Fix partition recovery with unavailable CPUs

[Guopeng Zhang <guopeng.zhang@linux.dev>]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

An invalid local partition can become valid again even when its
exclusive CPUs are no longer available from its parent.

This can be reproduced on a cgroup v2 system with CPUs 0-7 online:

    cd /sys/fs/cgroup
    echo +cpuset > cgroup.subtree_control
    mkdir A
    echo 1-4 > A/cpuset.cpus
    echo 1-4 > A/cpuset.cpus.exclusive
    echo root > A/cpuset.cpus.partition
    echo +cpuset > A/cgroup.subtree_control
    mkdir A/B
    echo 3-4 > A/B/cpuset.cpus
    echo 3-4 > A/B/cpuset.cpus.exclusive
    echo root > A/B/cpuset.cpus.partition
    echo 1-2 > A/cpuset.cpus.exclusive
    cat A/B/cpuset.cpus.partition
    root
    cat A/B/cpuset.cpus.exclusive.effective
    cat A/cpuset.cpus.effective
    1-2
    cat A/B/cpuset.cpus.effective
    1-2

B is changed back to a valid root even though its exclusive CPUs are
no longer available and cpuset.cpus.exclusive.effective is empty. B
should remain invalid until A can provide its exclusive CPUs again.

Shrinking A's exclusive CPUs invalidates B in
compute_partition_effective_cpumask(). The subsequent walk in
update_cpumasks_hier() tries to recover B, but
update_parent_effective_cpumask() skips the recovery checks when xcpus
is not a subset of parent->effective_xcpus. Since part_error is still
zero, B is changed back to a valid partition.

Set PERR_INVCPUS when the subset check fails so that the partition stays
invalid. New test cases are added to test_cpuset_prs.sh for removing and
restoring the parent's exclusive CPUs.

Fixes: 0c7f293efc87 ("cgroup/cpuset: Add cpuset.cpus.exclusive.effective for v2")
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
 kernel/cgroup/cpuset.c                            | 8 ++++++--
 tools/testing/selftests/cgroup/test_cpuset_prs.sh | 4 ++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 585a982249ad..be80763953ed 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1986,12 +1986,16 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 				cpumask_copy(tmp->addmask, cs->effective_xcpus);
 				adding = true;
 			}
-		} else if (is_partition_invalid(cs) && !cpumask_empty(xcpus) &&
-			   cpumask_subset(xcpus, parent->effective_xcpus)) {
+		} else if (is_partition_invalid(cs) && !cpumask_empty(xcpus)) {
 			struct cgroup_subsys_state *css;
 			struct cpuset *child;
 			bool exclusive = true;
 
+			if (!cpumask_subset(xcpus, parent->effective_xcpus)) {
+				part_error = PERR_INVCPUS;
+				goto write_error;
+			}
+
 			/*
 			 * Convert invalid partition to valid has to pass the
 			 * cpu exclusivity test as well as the housekeeping
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index d04aad697b96..f1d517647ade 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -427,6 +427,10 @@ TEST_MATRIX=(
 	# cpuset.cpus is allowed, but those extra CPUs are ignored.
 	"  C2-3:P1   C3:P1  .      .      .     C2-4    .      .     0 A1:|A2:2-3 A1:P1|A2:P1"
 
+	# A child cannot recover until its exclusive CPUs are available again.
+	" CX1-4:P1 CX3-4:P1 .     .     X1-2    .      .      .     0 A1:1-2|A2:1-2|XA2: A1:P1|A2:P-1"
+	" CX1-4:P1 CX3-4:P1 .     .  X1-2:X1-4 .      .      .     0 A1:1-2|A2:3-4|XA2:3-4 A1:P1|A2:P1"
+
 	# Taking away all CPUs from parent or itself if there are tasks
 	# will make the partition invalid.
 	"  C2-3:P1   C3:P1  .      .      T     C2-3    .      .     0 A1:2-3|A2:2-3 A1:P1|A2:P-1"
-- 
2.43.0