On Wed, Apr 11, 2018 at 04:19:18PM +0300, Sagi Grimberg wrote:
>
>> static void __blk_mq_requeue_request(struct request *rq)
>> {
>> struct request_queue *q = rq->q;
>> + enum mq_rq_state old_state = blk_mq_rq_state(rq);
>> blk_mq_put_driver_tag(rq);
>> trace_block_rq_requeue(q, rq);
>> wbt_requeue(q->rq_wb, &rq->issue_stat);
>> - if (blk_mq_rq_state(rq) != MQ_RQ_IDLE) {
>> - blk_mq_rq_update_state(rq, MQ_RQ_IDLE);
>> + if (old_state != MQ_RQ_IDLE) {
>> + if (!blk_mq_change_rq_state(rq, old_state, MQ_RQ_IDLE))
>> + WARN_ON_ONCE(true);
>> if (q->dma_drain_size && blk_rq_bytes(rq))
>> rq->nr_phys_segments--;
>> }
>
> Can you explain why was old_state kept as a local variable?
As it was me who added this: just to not read it again as no one
else can change the state at this point.
>> +static inline bool blk_mq_change_rq_state(struct request *rq,
>> + enum mq_rq_state old_state,
>> + enum mq_rq_state new_state)
>> {
>> - u64 old_val = READ_ONCE(rq->gstate);
>> - u64 new_val = (old_val & ~MQ_RQ_STATE_MASK) | state;
>> -
>> - if (state == MQ_RQ_IN_FLIGHT) {
>> - WARN_ON_ONCE((old_val & MQ_RQ_STATE_MASK) != MQ_RQ_IDLE);
>> - new_val += MQ_RQ_GEN_INC;
>> - }
>> + unsigned long old_val = (READ_ONCE(rq->__deadline) & ~RQ_STATE_MASK) |
>> + old_state;
>> + unsigned long new_val = (old_val & ~RQ_STATE_MASK) | new_state;
>> - /* avoid exposing interim values */
>> - WRITE_ONCE(rq->gstate, new_val);
>> + return cmpxchg(&rq->__deadline, old_val, new_val) == old_val;
>> }
>
> Can you explain why this takes the old_state of the request?
>
> Otherwise this looks good to me,
Because that is how cmpxchg works?
