Re: [PATCH v5 2/6] nbd: make sure request completion won't concurrent
On Thu, Sep 09, 2021 at 10:12:52PM +0800, Yu Kuai wrote:
> 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 04861b585b62..550c8dc438ac 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);
> @@ -842,7 +846,10 @@ static bool nbd_clear_req(struct request *req, void *data, bool reserved)
>
> mutex_lock(&cmd->lock);
> cmd->status = BLK_STS_IOERR;
> - __clear_bit(NBD_CMD_INFLIGHT, &cmd->flags);
> + if (!__test_and_clear_bit(NBD_CMD_INFLIGHT, &cmd->flags)) {
> + mutex_unlock(&cmd->lock);
> + return true;
> + }
> mutex_unlock(&cmd->lock);
If this request has completed from other code paths, ->status shouldn't be
updated here, maybe it is done successfully.
--
Ming
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