[PATCH v2 03/10] perf/arm-cmn: Avoid placing cpumask var on stack

[Dawei Li]

For CONFIG_CPUMASK_OFFSTACK=y kernel, explicit allocation of cpumask
variable on stack is not recommended since it can cause potential stack
overflow.

Instead, kernel code should always use *cpumask_var API(s) to allocate
cpumask var in config-neutral way, leaving allocation strategy to
CONFIG_CPUMASK_OFFSTACK.

But dynamic allocation in cpuhp's teardown callback is somewhat problematic
for if allocation fails(which is unlikely but still possible):
- If -ENOMEM is returned to caller, kernel crashes for non-bringup
  teardown;
- If callback pretends nothing happened and returns 0 to caller, it may
  trap system into an in-consisitent/compromised state;

Use newly-introduced cpumask_any_and_but() to address all issues above.
It eliminates usage of temporary cpumask var in generic way, no matter how
the cpumask var is allocated.

Suggested-by: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Dawei Li <dawei.li@xxxxxxxxxxxx>
---
 drivers/perf/arm-cmn.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
index 7ef9c7e4836b..6bfb0c4a1287 100644
--- a/drivers/perf/arm-cmn.c
+++ b/drivers/perf/arm-cmn.c
@@ -1950,20 +1950,20 @@ static int arm_cmn_pmu_offline_cpu(unsigned int cpu, struct hlist_node *cpuhp_no
 	struct arm_cmn *cmn;
 	unsigned int target;
 	int node;
-	cpumask_t mask;
 
 	cmn = hlist_entry_safe(cpuhp_node, struct arm_cmn, cpuhp_node);
 	if (cpu != cmn->cpu)
 		return 0;
 
 	node = dev_to_node(cmn->dev);
-	if (cpumask_and(&mask, cpumask_of_node(node), cpu_online_mask) &&
-	    cpumask_andnot(&mask, &mask, cpumask_of(cpu)))
-		target = cpumask_any(&mask);
-	else
+
+	target = cpumask_any_and_but(cpumask_of_node(node), cpu_online_mask, cpu);
+	if (target >= nr_cpu_ids)
 		target = cpumask_any_but(cpu_online_mask, cpu);
+
 	if (target < nr_cpu_ids)
 		arm_cmn_migrate(cmn, target);
+
 	return 0;
 }
 
-- 
2.27.0