mm: lru_cache_disable: replace work queue synchronization with synchronize_rcu

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 d479960e ("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.

Link: https://lkml.kernel.org/r/Yin7hDxdt0s/x+fp@fuller.cnet


Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>