[PATCH 1/5] writeback: laptop_mode_timer_fn() needs rcu_read_lock() around bdi_writeback iteration

From: Tejun Heo
Date: Tue Sep 29 2015 - 12:48:17 EST

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@xxxxxxxxxx>
Fixes: a06fd6b10228 ("writeback: make laptop_mode_timer_fn() handle multiple bdi_writeback's")
mm/page-writeback.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 0a931cd..902e5f2 100644
--- 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))

+ 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,
+ rcu_read_unlock();


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