block: assign correct tag before doing prefetch of request

Ensure that current tag is correctly assigned before attempting
to prefetch the first cacheline of the request.

Fixes: 92aff191cc5b ("block: prefetch request to be initialized")
Reported-and-tested-by: syzbot+cd20829ac44b92bf6ed0@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 221d1b7d10d663f6a11272d06f112efa17e3e7cc..4787d5b74aa3c6ce2108a4ee0e7b381dc3bad0c3 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -405,8 +405,8 @@ __blk_mq_alloc_requests_batch(struct blk_mq_alloc_data *data,
        for (i = 0; tag_mask; i++) {
                if (!(tag_mask & (1UL << i)))
                        continue;
-               prefetch(tags->static_rqs[tag]);
                tag = tag_offset + i;
+               prefetch(tags->static_rqs[tag]);
                tag_mask &= ~(1UL << i);
                rq = blk_mq_rq_ctx_init(data, tags, tag, alloc_time_ns);
                rq_list_add(data->cached_rq, rq);