diff mbox series

[24/41] cgroup: use irqsave in cgroup_rstat_flush_locked().

Message ID 20220323230630.8C004C340E8@smtp.kernel.org (mailing list archive)
State New
Headers show
Series [01/41] proc: alloc PATH_MAX bytes for /proc/${pid}/fd/ symlinks | expand

Commit Message

Andrew Morton March 23, 2022, 11:06 p.m. UTC 
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: 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>
---

 kernel/cgroup/rstat.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)
diff mbox series

Patch

--- a/kernel/cgroup/rstat.c~cgroup-use-irqsave-in-cgroup_rstat_flush_locked
+++ a/kernel/cgroup/rstat.c
@@ -153,8 +153,17 @@  static void cgroup_rstat_flush_locked(st
 		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(st
 				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() ||