[patch v5] mm: lru_cache_disable: replace work queue synchronization with synchronize_rcu

[Marcelo Tosatti <mtosatti@redhat.com>]

On systems that run FIFO:1 applications that busy loop,
any SCHED_OTHER task that attempts to execute
on such a CPU (such as work threads) will not
be scheduled, which leads to system hangs.

Commit d479960e44f27e0e52ba31b21740b703c538027c ("mm: disable LRU
pagevec during the migration temporarily") relies on
queueing work items on all online CPUs to ensure visibility
of lru_disable_count.

To fix this, replace the usage of work items with synchronize_rcu,
which provides the same guarantees.

Readers of lru_disable_count are protected by either disabling
preemption or rcu_read_lock:

preempt_disable, local_irq_disable  [bh_lru_lock()]
rcu_read_lock                       [rt_spin_lock CONFIG_PREEMPT_RT]
preempt_disable                     [local_lock !CONFIG_PREEMPT_RT]

Since v5.1 kernel, synchronize_rcu() is guaranteed to wait on
preempt_disable() regions of code. So any CPU which sees
lru_disable_count = 0 will have exited the critical
section when synchronize_rcu() returns.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Acked-by: Minchan Kim <minchan@kernel.org>

---
 
 v5: changelog improvements  		(Andrew Morton)
 v4: improve comment clarity, mention synchronize_rcu guarantees
     on v5.1				(Andrew Morton /
						 Paul E. McKenney)
 v3: update stale comment		(Nicolas Saenz Julienne)
 v2: rt_spin_lock calls rcu_read_lock, no need
 to add it before local_lock on swap.c	(Nicolas Saenz Julienne) 

diff --git a/mm/swap.c b/mm/swap.c
index bcf3ac288b56..b5ee163daa66 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -831,8 +831,7 @@ inline void __lru_add_drain_all(bool force_all_cpus)
 	for_each_online_cpu(cpu) {
 		struct work_struct *work = &per_cpu(lru_add_drain_work, cpu);
 
-		if (force_all_cpus ||
-		    pagevec_count(&per_cpu(lru_pvecs.lru_add, cpu)) ||
+		if (pagevec_count(&per_cpu(lru_pvecs.lru_add, cpu)) ||
 		    data_race(pagevec_count(&per_cpu(lru_rotate.pvec, cpu))) ||
 		    pagevec_count(&per_cpu(lru_pvecs.lru_deactivate_file, cpu)) ||
 		    pagevec_count(&per_cpu(lru_pvecs.lru_deactivate, cpu)) ||
@@ -876,15 +875,21 @@ atomic_t lru_disable_count = ATOMIC_INIT(0);
 void lru_cache_disable(void)
 {
 	atomic_inc(&lru_disable_count);
-#ifdef CONFIG_SMP
 	/*
-	 * lru_add_drain_all in the force mode will schedule draining on
-	 * all online CPUs so any calls of lru_cache_disabled wrapped by
-	 * local_lock or preemption disabled would be ordered by that.
-	 * The atomic operation doesn't need to have stronger ordering
-	 * requirements because that is enforced by the scheduling
-	 * guarantees.
+	 * Readers of lru_disable_count are protected by either disabling
+	 * preemption or rcu_read_lock:
+	 *
+	 * preempt_disable, local_irq_disable  [bh_lru_lock()]
+	 * rcu_read_lock		       [rt_spin_lock CONFIG_PREEMPT_RT]
+	 * preempt_disable		       [local_lock !CONFIG_PREEMPT_RT]
+	 *
+	 * Since v5.1 kernel, synchronize_rcu() is guaranteed to wait on
+	 * preempt_disable() regions of code. So any CPU which sees
+	 * lru_disable_count = 0 will have exited the critical
+	 * section when synchronize_rcu() returns.
 	 */
+	synchronize_rcu();
+#ifdef CONFIG_SMP
 	__lru_add_drain_all(true);
 #else
 	lru_add_and_bh_lrus_drain();