From 2971c35f35886b87af54675313a2afef937c1b0c Mon Sep 17 00:00:00 2001 From: Alexander Gordeev Date: Thu, 12 Jun 2014 17:05:37 +0200 Subject: [PATCH] blk-mq: bitmap tag: fix race on blk_mq_bitmap_tags::wake_cnt This piece of code in bt_clear_tag() function is racy: bs = bt_wake_ptr(bt); if (bs && atomic_dec_and_test(&bs->wait_cnt)) { atomic_set(&bs->wait_cnt, bt->wake_cnt); wake_up(&bs->wait); } Since nothing prevents bt_wake_ptr() from returning the very same 'bs' address on multiple CPUs, the following scenario is possible: CPU1 CPU2 ---- ---- 0. bs = bt_wake_ptr(bt); bs = bt_wake_ptr(bt); 1. atomic_dec_and_test(&bs->wait_cnt) 2. atomic_dec_and_test(&bs->wait_cnt) 3. atomic_set(&bs->wait_cnt, bt->wake_cnt); If the decrement in [1] yields zero then for some amount of time the decrement in [2] results in a negative/overflow value, which is not expected. The follow-up assignment in [3] overwrites the invalid value with the batch value (and likely prevents the issue from being severe) which is still incorrect and should be a lesser. Cc: Ming Lei Cc: Jens Axboe Signed-off-by: Alexander Gordeev Signed-off-by: Jens Axboe --- block/blk-mq-tag.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c index 6deb13055490..08fc6716d362 100644 --- a/block/blk-mq-tag.c +++ b/block/blk-mq-tag.c @@ -344,6 +344,7 @@ static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag) { const int index = TAG_TO_INDEX(bt, tag); struct bt_wait_state *bs; + int wait_cnt; /* * The unlock memory barrier need to order access to req in free @@ -352,10 +353,19 @@ static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag) clear_bit_unlock(TAG_TO_BIT(bt, tag), &bt->map[index].word); bs = bt_wake_ptr(bt); - if (bs && atomic_dec_and_test(&bs->wait_cnt)) { - atomic_set(&bs->wait_cnt, bt->wake_cnt); + if (!bs) + return; + + wait_cnt = atomic_dec_return(&bs->wait_cnt); + if (wait_cnt == 0) { +wake: + atomic_add(bt->wake_cnt, &bs->wait_cnt); bt_index_atomic_inc(&bt->wake_index); wake_up(&bs->wait); + } else if (wait_cnt < 0) { + wait_cnt = atomic_inc_return(&bs->wait_cnt); + if (!wait_cnt) + goto wake; } } -- 2.20.1