From: Joseph Qi <qijiang...@alibaba-inc.com>

Since throtl_rb_first may return NULL, so it should check tg first and
then use it.

Signed-off-by: Joseph Qi <qijiang...@alibaba-inc.com>
---
 block/blk-throttle.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index a7285bf..c1d2c5b 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1201,7 +1201,7 @@ static int throtl_select_dispatch(struct 
throtl_service_queue *parent_sq)
 
        while (1) {
                struct throtl_grp *tg = throtl_rb_first(parent_sq);
-               struct throtl_service_queue *sq = &tg->service_queue;
+               struct throtl_service_queue *sq;
 
                if (!tg)
                        break;
@@ -1213,6 +1213,7 @@ static int throtl_select_dispatch(struct 
throtl_service_queue *parent_sq)
 
                nr_disp += throtl_dispatch_tg(tg);
 
+               sq = &tg->service_queue;
                if (sq->nr_queued[0] || sq->nr_queued[1])
                        tg_update_disptime(tg);
 
-- 
1.9.4

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