[PATCH v2] blk-cgroup: Replace u64_sync with blkg_stat_lock for stats update

[boy . wu]

From: Boy Wu <boy.wu@xxxxxxxxxxxx>

In 32bit SMP systems, if multiple CPUs call blkcg_print_stat,
which 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.

Thus use blkg_stat_lock to replace u64_sync.

Fixes: ef45fe470e1e ("blk-cgroup: show global disk stats in root cgroup io.stat")
Signed-off-by: Boy Wu <boy.wu@xxxxxxxxxxxx>
---
Change in v2:
 - update commit message
 - Remove u64_sync
 - Replace spin_lock_irq with guard statement
 - Replace blkg->q->queue_lock with blkg_stat_lock
---
 block/blk-cgroup.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 85b3b9051455..18b47ee1a640 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -952,7 +952,6 @@ static void blkcg_fill_root_iostats(void)
 		struct blkcg_gq *blkg = bdev->bd_disk->queue->root_blkg;
 		struct blkg_iostat tmp;
 		int cpu;
-		unsigned long flags;
 
 		memset(&tmp, 0, sizeof(tmp));
 		for_each_possible_cpu(cpu) {
@@ -974,9 +973,10 @@ static void blkcg_fill_root_iostats(void)
 				cpu_dkstats->sectors[STAT_DISCARD] << 9;
 		}
 
-		flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);
+#if BITS_PER_LONG == 32
+		guard(raw_spinlock_irqsave)(&blkg_stat_lock);
+#endif
 		blkg_iostat_set(&blkg->iostat.cur, &tmp);
-		u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);
 	}
 }
 
-- 
2.18.0