[PATCH-next 3/6] cgroup/cpuset: Fix incorrect effective_cpus bug in update_cpumasks_hier()

[Waiman Long]

With the introduction of cpuset.cpus.exclusive, the computation of
effective_cpus differs depending on if a cpuset is a valid partition
root or not.

In update_cpumasks_hier(), effective_cpus is computed early
to check if the current subtree can be skipped. Later on
update_parent_effective_cpumask() may be called which can change the
partition state from valid to invalid or vice versa. If the partition
state changes, the previously computed effective_cpus may no longer be
valid. Fix that by recomputing effective_cpus again when that happens.

Also add a new test case into test_cpuset_prs.sh to verify this change.

Fixes: e2ffe502ba45 ("cgroup/cpuset: Add cpuset.cpus.exclusive for v2")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/cgroup/cpuset.c                            | 10 ++++++++--
 tools/testing/selftests/cgroup/test_cpuset_prs.sh |  3 +++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index db6e4663f4be..7b9062326f35 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2316,10 +2316,16 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
 		if (update_parent) {
 			update_parent_effective_cpumask(cp, partcmd_update, NULL, tmp);
 			/*
-			 * The cpuset partition_root_state may become
-			 * invalid. Capture it.
+			 * Capture current partition_root_state as it may have
+			 * changed. Recompute effective cpumask if changed.
 			 */
 			new_prs = cp->partition_root_state;
+			if (new_prs != old_prs) {
+				if (new_prs > 0)
+					compute_partition_effective_cpumask(cp, tmp->new_cpus);
+				else
+					compute_effective_cpumask(tmp->new_cpus, cp, parent);
+			}
 		}
 
 		spin_lock_irq(&callback_lock);
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index 47791591002f..8595b6b4b7bf 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -364,6 +364,9 @@ TEST_MATRIX=(
 	" C1-4:X2-4 C2-4:X3-4:P1 C3-4:X4:P2 \
 				   .      .      X5      .      .    0 A1:1-4|A2:2-4|A3:3-4 \
 								       A1:P0|A2:P-1|A3:P-2 ."
+	" C1-5:X3-5:P2 C3-5:X4-5:P2 C4-5:X5:P2 \
+				   .    X1-4     .       .      .    0 A1:1-3|A2:4|A3:4 \
+								       A1:P2|A2:P2|A3:P-2 1-4"
 	" C1-4:X2-4 C1-4:X3-4:P1 C3-4:X4:P2 \
 				   .      .      .      X1      .    0 A1:1-2|A2:3-4|A3:3-4 \
 								       A1:P0|A2:P1|A3:P-2 ."
-- 
2.55.0