[PATCH] blk-cgroup: add spin_lock for u64_stats_update

[boy.wu]

From: Boy Wu <boy.wu at mediatek.com>

In 32bit SMP systems, if the system is stressed on the sys node
by processes, it may cause blkcg_fill_root_iostats to have a concurrent
problem on the seqlock in u64_stats_update, which will cause a deadlock 
on u64_stats_fetch_begin in blkcg_print_one_stat.

To prevent this problem, add spin_locks.

Fixes: ef45fe470e1e ("blk-cgroup: show global disk stats in root cgroup io.stat")
Signed-off-by: Boy Wu <boy.wu at mediatek.com>
---
 block/blk-cgroup.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 37e6cc91d576..a633b7431e91 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1134,9 +1134,15 @@ static void blkcg_fill_root_iostats(void)
 				cpu_dkstats->sectors[STAT_DISCARD] << 9;
 		}
 
+#if BITS_PER_LONG == 32
+		spin_lock_irq(&blkg->q->queue_lock);
+#endif
 		flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);
 		blkg_iostat_set(&blkg->iostat.cur, &tmp);
 		u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);
+#if BITS_PER_LONG == 32
+		spin_unlock_irq(&blkg->q->queue_lock);
+#endif
 	}
 }
 
-- 
2.18.0