writeback: laptop_mode_timer_fn() needs rcu_read_lock() around bdi_writeback iteration

laptop_mode_timer_fn() was using bdi_for_each_wb() without the
required RCU locking leading to the following warning.

 WARNING: CPU: 0 PID: 0 at include/linux/backing-dev.h:415 laptop_mode_timer_fn+0x106/0x170()
 ...
 Call Trace:
  <IRQ>  [<ffffffff81480cdc>] dump_stack+0x4e/0x82
  [<ffffffff81051912>] warn_slowpath_common+0x82/0xc0
  [<ffffffff81051a0a>] warn_slowpath_null+0x1a/0x20
  [<ffffffff8115f0e6>] laptop_mode_timer_fn+0x106/0x170
  [<ffffffff810ca8e3>] call_timer_fn+0xb3/0x2f0
  [<ffffffff810cad25>] run_timer_softirq+0x205/0x370
  [<ffffffff81056854>] __do_softirq+0xd4/0x460
  [<ffffffff81056d69>] irq_exit+0x89/0xa0
  [<ffffffff8185a892>] smp_apic_timer_interrupt+0x42/0x50
  [<ffffffff81858a44>] apic_timer_interrupt+0x84/0x90
 ...

Fix it by adding rcu_read_lock() around the iteration.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: a06fd6b10228 ("writeback: make laptop_mode_timer_fn() handle multiple bdi_writeback's")
Reviewed-by: Jan Kara <jack@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 0a931cdd4f6baaa96cdfab2e0dd668c0abef8809..902e5f215e57ec61d120dc213c1e2fac5400df0b 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1965,10 +1965,12 @@ void laptop_mode_timer_fn(unsigned long data)
        if (!bdi_has_dirty_io(&q->backing_dev_info))
                return;
 
+       rcu_read_lock();
        bdi_for_each_wb(wb, &q->backing_dev_info, &iter, 0)
                if (wb_has_dirty_io(wb))
                        wb_start_writeback(wb, nr_pages, true,
                                           WB_REASON_LAPTOP_TIMER);
+       rcu_read_unlock();
 }
 
 /*