cgroup: use irqsave in cgroup_rstat_flush_locked().

All callers of cgroup_rstat_flush_locked() acquire cgroup_rstat_lock
either with spin_lock_irq() or spin_lock_irqsave().
cgroup_rstat_flush_locked() itself acquires cgroup_rstat_cpu_lock which
is a raw_spin_lock.  This lock is also acquired in
cgroup_rstat_updated() in IRQ context and therefore requires _irqsave()
locking suffix in cgroup_rstat_flush_locked().

Since there is no difference between spin_lock_t and raw_spin_lock_t on
!RT lockdep does not complain here.  On RT lockdep complains because the
interrupts were not disabled here and a deadlock is possible.

Acquire the raw_spin_lock_t with disabled interrupts.

Link: https://lkml.kernel.org/r/20220301122143.1521823-2-bigeasy@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Zefan Li <lizefan.x@bytedance.com>
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: cgroup: add a comment to cgroup_rstat_flush_locked().

Add a comment why spin_lock_irq() -> raw_spin_lock_irqsave() is needed.

Link: https://lkml.kernel.org/r/Yh+DOK73hfVV5ThX@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 9d331ba44870ab4715ec5444977a8441ec8af655..ba7a660184e4137c2d81743527e9df8ef954bf7d 100644 (file)
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -153,8 +153,17 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
                raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock,
                                                       cpu);
                struct cgroup *pos = NULL;
+               unsigned long flags;
 
-               raw_spin_lock(cpu_lock);
+               /*
+                * 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;
 
@@ -166,7 +175,7 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
                                css->ss->css_rstat_flush(css, cpu);
                        rcu_read_unlock();
                }
-               raw_spin_unlock(cpu_lock);
+               raw_spin_unlock_irqrestore(cpu_lock, flags);
 
                /* if @may_sleep, play nice and yield if necessary */
                if (may_sleep && (need_resched() ||