On Thu, Oct 27, 2016 at 6:53 AM, Bart Van Assche
<bart.vanassche@xxxxxxxxxxx> wrote:
> blk_mq_quiesce_queue() waits until ongoing .queue_rq() invocations
> have finished. This function does *not* wait until all outstanding
> requests have finished (this means invocation of request.end_io()).
> The algorithm used by blk_mq_quiesce_queue() is as follows:
> * Hold either an RCU read lock or an SRCU read lock around
> .queue_rq() calls. The former is used if .queue_rq() does not
> block and the latter if .queue_rq() may block.
> * blk_mq_quiesce_queue() calls synchronize_srcu() or
> synchronize_rcu() to wait for .queue_rq() invocations that
> started before blk_mq_quiesce_queue() was called.
> * The blk_mq_hctx_stopped() calls that control whether or not
> .queue_rq() will be called are called with the (S)RCU read lock
> held. This is necessary to avoid race conditions against
> the "blk_mq_stop_hw_queues(q); blk_mq_quiesce_queue(q);"
> sequence from another thread.
>
> Signed-off-by: Bart Van Assche <bart.vanassche@xxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Ming Lei <tom.leiming@xxxxxxxxx>
> Cc: Hannes Reinecke <hare@xxxxxxxx>
> Cc: Johannes Thumshirn <jthumshirn@xxxxxxx>
> ---
> block/Kconfig | 1 +
> block/blk-mq.c | 69 +++++++++++++++++++++++++++++++++++++++++++++-----
> include/linux/blk-mq.h | 3 +++
> include/linux/blkdev.h | 1 +
> 4 files changed, 67 insertions(+), 7 deletions(-)
>
> diff --git a/block/Kconfig b/block/Kconfig
> index 1d4d624..0562ef9 100644
> --- a/block/Kconfig
> +++ b/block/Kconfig
> @@ -5,6 +5,7 @@ menuconfig BLOCK
> bool "Enable the block layer" if EXPERT
> default y
> select SBITMAP
> + select SRCU
> help
> Provide block layer support for the kernel.
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 0cf21c2..4945437 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -115,6 +115,31 @@ void blk_mq_unfreeze_queue(struct request_queue *q)
> }
> EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue);
>
> +/**
> + * blk_mq_quiesce_queue() - wait until all ongoing queue_rq calls have finished
> + * @q: request queue.
> + *
> + * Note: this function does not prevent that the struct request end_io()
> + * callback function is invoked. Additionally, it is not prevented that
> + * new queue_rq() calls occur unless the queue has been stopped first.
> + */
> +void blk_mq_quiesce_queue(struct request_queue *q)
> +{
> + struct blk_mq_hw_ctx *hctx;
> + unsigned int i;
> + bool rcu = false;
Before synchronizing SRCU/RCU, we have to set a per-hctx flag
or per-queue flag to block comming .queue_rq(), seems I mentioned
that before:
https://www.spinics.net/lists/linux-rdma/msg41389.html
> +
> + queue_for_each_hw_ctx(q, hctx, i) {
> + if (hctx->flags & BLK_MQ_F_BLOCKING)
> + synchronize_srcu(&hctx->queue_rq_srcu);
> + else
> + rcu = true;
> + }
> + if (rcu)
> + synchronize_rcu();
> +}
> +EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue);
> +
> void blk_mq_wake_waiters(struct request_queue *q)
> {
> struct blk_mq_hw_ctx *hctx;
> @@ -778,7 +803,7 @@ static inline unsigned int queued_to_index(unsigned int queued)
> * of IO. In particular, we'd like FIFO behaviour on handling existing
> * items on the hctx->dispatch list. Ignore that for now.
> */
> -static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
> +static void blk_mq_process_rq_list(struct blk_mq_hw_ctx *hctx)
> {
> struct request_queue *q = hctx->queue;
> struct request *rq;
> @@ -790,9 +815,6 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
> if (unlikely(blk_mq_hctx_stopped(hctx)))
> return;
>
> - WARN_ON(!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) &&
> - cpu_online(hctx->next_cpu));
> -
> hctx->run++;
>
> /*
> @@ -883,6 +905,24 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
> }
> }
>
> +static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
> +{
> + int srcu_idx;
> +
> + WARN_ON(!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) &&
> + cpu_online(hctx->next_cpu));
> +
> + if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
> + rcu_read_lock();
> + blk_mq_process_rq_list(hctx);
> + rcu_read_unlock();
> + } else {
> + srcu_idx = srcu_read_lock(&hctx->queue_rq_srcu);
> + blk_mq_process_rq_list(hctx);
> + srcu_read_unlock(&hctx->queue_rq_srcu, srcu_idx);
> + }
> +}
> +
> /*
> * It'd be great if the workqueue API had a way to pass
> * in a mask and had some smarts for more clever placement.
> @@ -1293,7 +1333,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
> const int is_flush_fua = bio->bi_opf & (REQ_PREFLUSH | REQ_FUA);
> struct blk_map_ctx data;
> struct request *rq;
> - unsigned int request_count = 0;
> + unsigned int request_count = 0, srcu_idx;
> struct blk_plug *plug;
> struct request *same_queue_rq = NULL;
> blk_qc_t cookie;
> @@ -1336,7 +1376,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
> blk_mq_bio_to_request(rq, bio);
>
> /*
> - * We do limited pluging. If the bio can be merged, do that.
> + * We do limited plugging. If the bio can be merged, do that.
> * Otherwise the existing request in the plug list will be
> * issued. So the plug list will have one request at most
> */
> @@ -1356,7 +1396,16 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
> blk_mq_put_ctx(data.ctx);
> if (!old_rq)
> goto done;
> - blk_mq_try_issue_directly(data.hctx, old_rq, &cookie);
> +
> + if (!(data.hctx->flags & BLK_MQ_F_BLOCKING)) {
> + rcu_read_lock();
> + blk_mq_try_issue_directly(data.hctx, old_rq, &cookie);
> + rcu_read_unlock();
> + } else {
> + srcu_idx = srcu_read_lock(&data.hctx->queue_rq_srcu);
> + blk_mq_try_issue_directly(data.hctx, old_rq, &cookie);
> + srcu_read_unlock(&data.hctx->queue_rq_srcu, srcu_idx);
> + }
> goto done;
> }
>
> @@ -1635,6 +1684,9 @@ static void blk_mq_exit_hctx(struct request_queue *q,
> if (set->ops->exit_hctx)
> set->ops->exit_hctx(hctx, hctx_idx);
>
> + if (hctx->flags & BLK_MQ_F_BLOCKING)
> + cleanup_srcu_struct(&hctx->queue_rq_srcu);
> +
> blk_mq_remove_cpuhp(hctx);
> blk_free_flush_queue(hctx->fq);
> sbitmap_free(&hctx->ctx_map);
> @@ -1715,6 +1767,9 @@ static int blk_mq_init_hctx(struct request_queue *q,
> flush_start_tag + hctx_idx, node))
> goto free_fq;
>
> + if (hctx->flags & BLK_MQ_F_BLOCKING)
> + init_srcu_struct(&hctx->queue_rq_srcu);
> +
> return 0;
>
> free_fq:
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index aa93000..61be48b 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -3,6 +3,7 @@
>
> #include <linux/blkdev.h>
> #include <linux/sbitmap.h>
> +#include <linux/srcu.h>
>
> struct blk_mq_tags;
> struct blk_flush_queue;
> @@ -35,6 +36,8 @@ struct blk_mq_hw_ctx {
>
> struct blk_mq_tags *tags;
>
> + struct srcu_struct queue_rq_srcu;
> +
> unsigned long queued;
> unsigned long run;
> #define BLK_MQ_MAX_DISPATCH_ORDER 7
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index c47c358..8259d87 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -824,6 +824,7 @@ extern void __blk_run_queue(struct request_queue *q);
> extern void __blk_run_queue_uncond(struct request_queue *q);
> extern void blk_run_queue(struct request_queue *);
> extern void blk_run_queue_async(struct request_queue *q);
> +extern void blk_mq_quiesce_queue(struct request_queue *q);
> extern int blk_rq_map_user(struct request_queue *, struct request *,
> struct rq_map_data *, void __user *, unsigned long,
> gfp_t);
> --
> 2.10.1
>
--
Ming Lei
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