linux-kernel - [PATCH] cgroup: use irqsave in cgroup_rstat_flush

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Date:   Tue,  3 Jul 2018 18:45:44 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     Tejun Heo <tj@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH] 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.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/cgroup/rstat.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index d503d1a9007c..63fc5e472c82 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -157,8 +157,9 @@ 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);
+		raw_spin_lock_irqsave(cpu_lock, flags);
 		while ((pos = cgroup_rstat_cpu_pop_updated(pos, cgrp, cpu))) {
 			struct cgroup_subsys_state *css;

@@ -170,7 +171,7 @@ 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() ||
-- 
2.18.0