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rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading 

The shrinker may run concurrently with callbacks (de-)offloading. As
such, calling rcu_nocb_lock() is very dangerous because it does a
conditional locking. The worst outcome is that rcu_nocb_lock() doesn't
lock but rcu_nocb_unlock() eventually unlocks, or the reverse, creating
an imbalance.

Fix this with protecting against (de-)offloading using the barrier mutex.
Although if the barrier mutex is contended, which should be rare, then
step aside so as not to trigger a mutex VS allocation
dependency chain.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Frederic Weisbecker 2023-03-29 18:02:00 +02:00 committed by Paul E. McKenney
parent ac9a78681b
commit 5c83cedbaa
1 changed files with 24 additions and 1 deletions
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25
kernel/rcu/tree_nocb.h
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@ -1336,13 +1336,33 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
unsigned long flags;
unsigned long count = 0;
/*
* Protect against concurrent (de-)offloading. Otherwise nocb locking
* may be ignored or imbalanced.
*/
if (!mutex_trylock(&rcu_state.barrier_mutex)) {
/*
* But really don't insist if barrier_mutex is contended since we
* can't guarantee that it will never engage in a dependency
* chain involving memory allocation. The lock is seldom contended
* anyway.
*/
return 0;
}
/* Snapshot count of all CPUs */
for_each_possible_cpu(cpu) {
struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
int _count = READ_ONCE(rdp->lazy_len);
int _count;
if (!rcu_rdp_is_offloaded(rdp))
continue;
_count = READ_ONCE(rdp->lazy_len);
if (_count == 0)
continue;
rcu_nocb_lock_irqsave(rdp, flags);
WRITE_ONCE(rdp->lazy_len, 0);
rcu_nocb_unlock_irqrestore(rdp, flags);
@ -1352,6 +1372,9 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
if (sc->nr_to_scan <= 0)
break;
}
mutex_unlock(&rcu_state.barrier_mutex);
return count ? count : SHRINK_STOP;
}