[PATCH-next 6/6] cgroup/cpuset: Prevent recursive call of remote_cpus_update() via update_cpumasks_hier()

[Waiman Long <longman@redhat.com>]

Guopeng reports that changing the cpumask of a remote partition root
may set the effective_cpus of the top_cpuset incorrectly. It is
due to the following call sequence:

  update_cpumask()
   -> partition_cpus_change()
    -> remote_cpus_update()
     -> update_sibling_cpumasks()
      -> update_cpumasks_hier()
       -> remote_cpus_update()

The second remote_cpus_update() call passes in the cs->effective_xcpus
instead of the trialcs->effective_xcpus of the first remote_cpus_update()
call.  As a result, the second call can revert what has been done in
the first call leading to incorrect effective_cpus of the top_cpuset
as well as subpartitions_cpus.

Fix that by adding a new remote_partition_updating flag into the cpuset
structure to indicate that a remote partition is being updated and
update_cpumasks_hier() should skip the whole subtree as update_cpumask()
or update_exclusive_cpumask() which calls partition_cpus_change()
will eventually calls update_cpumasks_hier() for that subtree anyway.

This new flag is currently set only in remote_cpus_update() to prevent
recursive call of remote_cpus_update() on the same cpuset.

A test case is added to test_cpuset_prs.sh to verify this change.

Reported-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Link: https://lore.kernel.org/lkml/20261010075947.333549-2-guopeng.zhang@linux.dev
Fixes: f62a5d39368e ("cgroup/cpuset: Remove remote_partition_check() & make update_cpumasks_hier() handle remote partition")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset-internal.h                   |  5 +++++
 kernel/cgroup/cpuset.c                            | 13 +++++++++----
 tools/testing/selftests/cgroup/test_cpuset_prs.sh |  1 +
 3 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
index 2e44dc8634dc..1ba94ffaff9d 100644
--- a/kernel/cgroup/cpuset-internal.h
+++ b/kernel/cgroup/cpuset-internal.h
@@ -161,6 +161,11 @@ struct cpuset {
 	 */
 	bool remote_partition;
 
+	/*
+	 * Set to true when a remote partition is undergoing update.
+	 */
+	bool remote_partition_updating;
+
 	/*
 	 * number of SCHED_DEADLINE tasks attached to this cpuset, so that we
 	 * know when to rebuild associated root domain bandwidth information.
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 6e1bef12f86d..952b31246d62 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1736,6 +1736,7 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
 			goto invalidate;
 	}
 
+	cs->remote_partition_updating = true;
 	spin_lock_irq(&callback_lock);
 	if (adding)
 		partition_xcpus_add(prs, NULL, tmp->addmask);
@@ -1757,6 +1758,7 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
 	 */
 	cpuset_update_tasks_cpumask(&top_cpuset, tmp->new_cpus);
 	update_sibling_cpumasks(&top_cpuset, NULL, tmp);
+	cs->remote_partition_updating = false;
 	return;
 
 invalidate:
@@ -2208,16 +2210,19 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
 		old_prs = new_prs = cp->partition_root_state;
 
 		/*
-		 * For child remote partition root (!= cs), we need to call
-		 * remote_cpus_update() if effective_xcpus will be changed.
-		 * Otherwise, we can skip the whole subtree.
+		 * For a child remote partition with remote_partition_updating
+		 * set, we can skip the whole subtree as the this function will
+		 * be called eventually. We can also skip the subtree if the
+		 * effective_xcpus isn't going to be changed. Otherwise, we
+		 * will have to call remote_cpus_update().
 		 *
 		 * remote_cpus_update() will reuse tmp->new_cpus only after
 		 * its value is being processed.
 		 */
 		if (remote && (cp != cs)) {
 			compute_excpus(cp, tmp->new_cpus);
-			if (cpumask_equal(cp->effective_xcpus, tmp->new_cpus)) {
+			if (cp->remote_partition_updating ||
+			    cpumask_equal(cp->effective_xcpus, tmp->new_cpus)) {
 				pos_css = css_rightmost_descendant(pos_css);
 				continue;
 			}
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index 5bc144836bdf..19c50819065c 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -347,6 +347,7 @@ TEST_MATRIX=(
 	"   C0-3    C1-3  C2-3    C4    X2-3   X2-3  X2-3:P2 P2:C4-5 0 A3:2-3|B1:4-5 A3:P2|B1:P2 2-5"
 	"  C4:X0-3  X1-3  X2-3     .      .      P2     .      .     0 A1:4|A2:1-3|A3:1-3 A2:P2 1-3"
 	"  C4:X0-3  X1-3  X2-3     .      .      .      P2     .     0 A1:4|A2:4|A3:2-3 A3:P2 2-3"
+	"   CX1-6  C1-4:P1  .    C4-5     .    C1-3     .      .     0 A1:4-6|A2:1-3|B1:4-5 A2:P1"
 
 	# Nested remote/local partition tests
 	"   C0-3    C1-3  C2-3   C4-5   X2-3  X2-3:P1   P2     P1    0 A1:0-1|A2:|A3:2-3|B1:4-5 \
-- 
2.55.0