[PATCH v6 2/6] nbd: make sure request completion won't concurrent

Yu Kuai
2021-09-15T08:05:58Z

commit cddce0116058 ("nbd: Aovid double completion of a request")
try to fix that nbd_clear_que() and recv_work() can complete a
request concurrently. However, the problem still exists:

t1 t2 t3

nbd_disconnect_and_put
flush_workqueue
recv_work
blk_mq_complete_request
blk_mq_complete_request_remote -> this is true
WRITE_ONCE(rq->state, MQ_RQ_COMPLETE)
blk_mq_raise_softirq
blk_done_softirq
blk_complete_reqs
nbd_complete_rq
blk_mq_end_request
blk_mq_free_request
WRITE_ONCE(rq->state, MQ_RQ_IDLE)
nbd_clear_que
blk_mq_tagset_busy_iter
nbd_clear_req
__blk_mq_free_request
blk_mq_put_tag
blk_mq_complete_request -> complete again

There are three places where request can be completed in nbd:
recv_work(), nbd_clear_que() and nbd_xmit_timeout(). Since they
all hold cmd->lock before completing the request, it's easy to
avoid the problem by setting and checking a cmd flag.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
---
drivers/block/nbd.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 23ded569e8d3..614c6ab2b8fe 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -406,7 +406,11 @@ static enum blk_eh_timer_return nbd_xmit_timeout(struct request *req,
if (!mutex_trylock(&cmd->lock))
return BLK_EH_RESET_TIMER;

- __clear_bit(NBD_CMD_INFLIGHT, &cmd->flags);
+ if (!__test_and_clear_bit(NBD_CMD_INFLIGHT, &cmd->flags)) {
+ mutex_unlock(&cmd->lock);
+ return BLK_EH_DONE;
+ }
+
if (!refcount_inc_not_zero(&nbd->config_refs)) {
cmd->status = BLK_STS_TIMEOUT;
mutex_unlock(&cmd->lock);
@@ -841,7 +845,10 @@ static bool nbd_clear_req(struct request *req, void *data, bool reserved)
return true;

mutex_lock(&cmd->lock);
- __clear_bit(NBD_CMD_INFLIGHT, &cmd->flags);
+ if (!__test_and_clear_bit(NBD_CMD_INFLIGHT, &cmd->flags)) {
+ mutex_unlock(&cmd->lock);
+ return true;
+ }
cmd->status = BLK_STS_IOERR;
mutex_unlock(&cmd->lock);

--
2.31.1