Add a comment why spin_lock_irq() -> raw_spin_lock_irqsave() is needed.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
On 2022-03-01 20:38:51 [-1000], Tejun Heo wrote:
> Hello,
Hello Tejun,
> Can you please add a comment explaining why irqsave is being used? As it
> stands, it just looks spurious.
Something like this?
kernel/cgroup/rstat.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 53b771c20ee50..ba7a660184e41 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -155,6 +155,14 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
struct cgroup *pos = NULL;
unsigned long flags;
+ /*
+ * The _irqsave() is needed because cgroup_rstat_lock is
+ * spinlock_t which is a sleeping lock on PREEMPT_RT. Acquiring
+ * this lock with the _irq() suffix only disables interrupts on
+ * a non-PREEMPT_RT kernel. The raw_spinlock_t below disables
+ * interrupts on both configurations. The _irqsave() ensures
+ * that interrupts are always disabled and later restored.
+ */
raw_spin_lock_irqsave(cpu_lock, flags);
while ((pos = cgroup_rstat_cpu_pop_updated(pos, cgrp, cpu))) {
struct cgroup_subsys_state *css;
--
2.35.1
