On 1/9/18 9:12 AM, Bart Van Assche wrote:
> On Mon, 2018-01-08 at 11:15 -0800, Tejun Heo wrote:
>> Currently, blk-mq protects only the issue path with RCU. This patch
>> puts the completion path under the same RCU protection. This will be
>> used to synchronize issue/completion against timeout by later patches,
>> which will also add the comments.
>>
>> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
>> ---
>> block/blk-mq.c | 5 +++++
>> 1 file changed, 5 insertions(+)
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index ddc9261..6741c3e 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -584,11 +584,16 @@ static void hctx_lock(struct blk_mq_hw_ctx *hctx, int *srcu_idx)
>> void blk_mq_complete_request(struct request *rq)
>> {
>> struct request_queue *q = rq->q;
>> + struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, rq->mq_ctx->cpu);
>> + int srcu_idx;
>>
>> if (unlikely(blk_should_fake_timeout(q)))
>> return;
>> +
>> + hctx_lock(hctx, &srcu_idx);
>> if (!blk_mark_rq_complete(rq))
>> __blk_mq_complete_request(rq);
>> + hctx_unlock(hctx, srcu_idx);
>> }
>> EXPORT_SYMBOL(blk_mq_complete_request);
>
> Hello Tejun,
>
> I'm concerned about the additional CPU cycles needed for the new blk_mq_map_queue()
> call, although I know this call is cheap. Would the timeout code really get that
> much more complicated if the hctx_lock() and hctx_unlock() calls would be changed
> into rcu_read_lock() and rcu_read_unlock() calls? Would it be sufficient if
> "if (has_rcu) synchronize_rcu();" would be changed into "synchronize_rcu();" in
> blk_mq_timeout_work()?
It's a non concern, imho. We do queue mapping all over the place for a variety
of reasons, it's total noise, especially since we're calling [s]rcu anyway
after that.
--
Jens Axboe
