On Tue, 2016-06-07 at 22:49 -0600, Jens Axboe wrote:

On 06/06/2016 03:21 PM, Christoph Hellwig wrote:

quoted

From: Ming Lin <redacted>

For some protocols like NVMe over Fabrics we need to be able to
send
initialization commands to a specific queue.

Based on an earlier patch from Christoph Hellwig [off-list ref].

Signed-off-by: Ming Lin <redacted>
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
  block/blk-mq.c         | 33 +++++++++++++++++++++++++++++++++
  include/linux/blk-mq.h |  2 ++
  2 files changed, 35 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 29cbc1b..7bb45ed 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -266,6 +266,39 @@ struct request *blk_mq_alloc_request(struct

request_queue *q, int rw,
  }
  EXPORT_SYMBOL(blk_mq_alloc_request);

+struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
int rw,
+		unsigned int flags, unsigned int hctx_idx)
+{
+	struct blk_mq_hw_ctx *hctx;
+	struct blk_mq_ctx *ctx;
+	struct request *rq;
+	struct blk_mq_alloc_data alloc_data;
+	int ret;
+
+	ret = blk_queue_enter(q, flags & BLK_MQ_REQ_NOWAIT);
+	if (ret)
+		return ERR_PTR(ret);
+
+	hctx = q->queue_hw_ctx[hctx_idx];
+	ctx = __blk_mq_get_ctx(q, cpumask_first(hctx->cpumask));
+
+	blk_mq_set_alloc_data(&alloc_data, q, flags, ctx, hctx);
+
+	rq = __blk_mq_alloc_request(&alloc_data, rw);
+	if (!rq && !(flags & BLK_MQ_REQ_NOWAIT)) {
+		__blk_mq_run_hw_queue(hctx);
+
+		rq =  __blk_mq_alloc_request(&alloc_data, rw);
+	}

Why are we duplicating this code here? If NOWAIT isn't set, then
we'll
always return a request. bt_get() will run the queue for us, if it
needs
to. blk_mq_alloc_request() does this too, and I'm guessing that code
was
just copied. I'll fix that up. Looks like this should just be:

	rq = __blk_mq_alloc_request(&alloc_data, rw);
	if (rq)
		return rq;

	blk_queue_exit(q);
	return ERR_PTR(-EWOULDBLOCK);

for this case.

Yes,

But the bt_get() reminds me that this patch actually has a problem.

blk_mq_alloc_request_hctx() ->
  __blk_mq_alloc_request() ->
    blk_mq_get_tag() -> 
      __blk_mq_get_tag() ->
        bt_get() ->
          blk_mq_put_ctx(data->ctx);

Here are blk_mq_get_ctx() and blk_mq_put_ctx().

static inline struct blk_mq_ctx *blk_mq_get_ctx(struct request_queue *q)
{       
        return __blk_mq_get_ctx(q, get_cpu());
} 

static inline void blk_mq_put_ctx(struct blk_mq_ctx *ctx)
{
        put_cpu();
}

blk_mq_alloc_request_hctx() calls __blk_mq_get_ctx() instead
of blk_mq_get_ctx(). Then reason is the "hctx" could belong to other
cpu. So blk_mq_get_ctx() doesn't work.

But then above put_cpu() in blk_mq_put_ctx() will trigger a WARNING
because we didn't do get_cpu() in blk_mq_alloc_request_hctx()