On Tue, Dec 24, 2013 at 11:30:58AM +0800, Ming Lei wrote: > Some drivers don't have LLDD, like null_blk, virtio-blk, ...
Well, they are the low-level driver in this case. > Actually the below block APIs > blk_get_request() > blk_put_request() > blk_execute_rq_nowait() > blk_register_queue() > blk_unregister_queue() > blk_insert_flush() > .... > > have been there to support MQ. All these are called by higher level code and not the driver, so we need a switch between MQ and non-MQ support somewhere. > One problem about the 2nd way is that MQ drivers may become a bit > ugly if the driver still need to support traditional request/bio mechanism. On the other hand drivers have no legitimate case for long-term parallel MQ/non-MQ code paths. The only exception is null_blk as a benchmarking vehicle, but we should not optimize for that. > >> + spin_unlock_irq(lock); > >> + } > > > > Why doesn't the mq case set QUEUE_FLAG_DEAD here? > > Looks DEAD flag is set to prevent request_fn from being called, > and there is no request_fn for MQ, so I don't set it and QUEUE_FLAG_DYING > has been checked in blk_mq_queue_enter() already. > > But we can do it easily. Do we have a per-context flag to prevent calling into the driver instead? Sorry, it's been a while that I last looked at the code, but anyone just looking over the difference will have the same question, so the answer needs to go into a comment. > > Also blk_execute_rq_nowait now needs to check the dying case for MQ as > > well. > > In this patch the dying flag is only checked in blk_mq_queue_enter() > which is called in allocating request path, so once the request is > allocated, we just simply wait for its completion in blk_mq_drain_queue(). > > Even for non-MQ case, looks blk_queue_dying(q) is checked to avoid > being freed early before running queue, I am not sure if it is good > because the I/O might or can have been completed. And it isn't a problem > for MQ because this request can't be reused when dying is set. Good. I think we'll want comments like that preferable in the code, but at very least in the commit log. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
