mm: fix lost kswapd wakeup in kswapd_stop()

commit 1c7e7f6c0703d03af6bcd5ccc11fc15d23e5ecbe upstream.

Offlining memory may block forever, waiting for kswapd() to wake up
because kswapd() does not check the event kthread->should_stop before
sleeping.

The proper pattern, from Documentation/memory-barriers.txt, is:

   ---  waker  ---
   event_indicated = 1;
   wake_up_process(event_daemon);

   ---  sleeper  ---
   for (;;) {
      set_current_state(TASK_UNINTERRUPTIBLE);
      if (event_indicated)
         break;
      schedule();
   }

   set_current_state() may be wrapped by:
      prepare_to_wait();

In the kswapd() case, event_indicated is kthread->should_stop.

  === offlining memory (waker) ===
   kswapd_stop()
      kthread_stop()
         kthread->should_stop = 1
         wake_up_process()
         wait_for_completion()

  ===  kswapd_try_to_sleep (sleeper) ===
   kswapd_try_to_sleep()
      prepare_to_wait()
           .
           .
      schedule()
           .
           .
      finish_wait()

The schedule() needs to be protected by a test of kthread->should_stop,
which is wrapped by kthread_should_stop().

Reproducer:
   Do heavy file I/O in background.
   Do a memory offline/online in a tight loop

Signed-off-by: Aaditya Kumar <aaditya.kumar@ap.sony.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Minchan Kim <minchan@kernel.org>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 72cf498f3a4e25b5f010338a919fdab32f012074..834211939d0e6eabbf3b2ce8d2b00f4cabf472b5 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2824,7 +2824,10 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, int classzone_idx)
                 * them before going back to sleep.
                 */
                set_pgdat_percpu_threshold(pgdat, calculate_normal_threshold);
-               schedule();
+
+               if (!kthread_should_stop())
+                       schedule();
+
                set_pgdat_percpu_threshold(pgdat, calculate_pressure_threshold);
        } else {
                if (remaining)