[PATCH v3 3/3] cgroup/cpuset: check sibling effective_xcpus in cpus_excl_conflict()

[Hui Peng]

In cpus_excl_conflict(), when a valid local partition A1 uses implicit
exclusive CPUs (cpuset.cpus set without cpuset.cpus.exclusive, so
sibling->exclusive_cpus is empty while sibling->effective_xcpus is
populated), a sibling cgroup B1 can still set cpuset.cpus.exclusive on
the same CPUs because cpus_excl_conflict() only checks
sibling->exclusive_cpus.

Fall back to sibling->effective_xcpus in cpus_excl_conflict() when
sibling->exclusive_cpus is empty, and add a regression test case to
tools/testing/selftests/cgroup/test_cpuset_prs.sh.

Tested in QEMU on Linux 7.3.0-rc3 using
tools/testing/selftests/cgroup/test_cpuset_prs.sh.

Fixes: 2a3602030d80 ("cgroup/cpuset: Don't invalidate sibling partitions on cpuset.cpus conflict")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@xxxxxxxxx>
---
Changes in v3:
- Split out from the validate_partition() fix into patch 3/3 and add the
  Fixes tag for commit 2a3602030d80, as pointed out by Guopeng Zhang.

 kernel/cgroup/cpuset.c                        | 5 ++++-
 tools/testing/selftests/cgroup/test_cpuset_prs.sh | 1 +
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7c5b4a8f1f3a..f46a51f07938 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -713,7 +713,10 @@ static inline bool cpus_excl_conflict(struct cpuset *trial, struct cpuset *sibli
 		return true;
 
 	/* Exclusive_cpus cannot intersect */
-	return cpumask_intersects(trial->exclusive_cpus, sibling->exclusive_cpus);
+	return cpumask_intersects(trial->exclusive_cpus,
+				  cpumask_empty(sibling->exclusive_cpus)
+				  ? sibling->effective_xcpus
+				  : sibling->exclusive_cpus);
 }
 
 static inline bool mems_excl_conflict(struct cpuset *cs1, struct cpuset *cs2)
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index f4d1822d4e21..f62155cf6162 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -445,6 +445,7 @@ TEST_MATRIX=(
 
 	# Changes to cpuset.cpus.exclusive that violate exclusivity rule is rejected
 	"   C0-3     .      .    C4-5   X0-3     .      .     X3-5   1 A1:0-3|B1:4-5"
+	"  C0-3:P1   .      .    C4-5     .      .      .     X3-5   1 A1:0-3|B1:4-5 A1:P1|B1:P0"
 
 	# cpuset.cpus.exclusive cannot be set to a superset of sibling's cpuset.cpus
 	"   C0-3     .      .    C4-5   X3-5     .      .      .     1 A1:0-3|B1:4-5"
-- 
2.49.0