Re: [PATCH] blk-mq: put driver tag when this request is completed

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jul 01, 2020 at 04:16:32PM +0200, Marek Szyprowski wrote:
> Hi Ming,
> 
> On 01.07.2020 15:45, Ming Lei wrote:
> > On Wed, Jul 01, 2020 at 03:01:03PM +0200, Marek Szyprowski wrote:
> >> On 29.06.2020 11:47, Ming Lei wrote:
> >>> It is natural to release driver tag when this request is completed by
> >>> LLD or device since its purpose is for LLD use.
> >>>
> >>> One big benefit is that the released tag can be re-used quicker since
> >>> bio_endio() may take too long.
> >>>
> >>> Meantime we don't need to release driver tag for flush request.
> >>>
> >>> Cc: Christoph Hellwig <hch@lst.de>
> >>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> >> This patch landed recently in linux-next as commit 36a3df5a4574. Sadly
> >> it causes a regression on one of my test systems (ARM 32bit, Samsung
> >> Exynos5422 SoC based Odroid XU3 board with eMMC). The system boots fine
> >> and then after a few seconds every executed command hangs. No
> >> panic/ops/any other message. I will try to provide more information asap
> >> I find something to share. Simple reverting it in linux-next is not
> >> possible due to dependencies.
> > What is the exact eMMC's driver code(include the host driver)?
> 
> dwmmc-exynos (drivers/mmc/host/dw_mmc-exynos.c)

Hi,

Just take a quick look at mmc code, there are only two req->tag
consumers:

1) cqhci_tag
cqhci_tag
	cqhci_request
		host->cqe_ops->cqe_request
			mmc_cqe_start_req
	cqhci_timeout

2) mmc_hsq_request
mmc_hsq_request
	host->cqe_ops->cqe_request
		mmc_cqe_start_req

mmc_cqe_start_req() is called before issuing this request to hardware,
so completion won't happen when the tag is used in mmc_cqe_start_req().

cqhci_timeout() may race with normal completion, however looks the
following code can handle the race correctly:

        spin_lock_irqsave(&cq_host->lock, flags);
        timed_out = slot->mrq == mrq;

So still no idea why the commit causes the trouble for mmc.

Do you know it is cqhci or mmc_hsh which works for dw_mmc-exynos?
And can you apply the following patch and see if warning can be
triggered?

diff --git a/drivers/mmc/host/cqhci.c b/drivers/mmc/host/cqhci.c
index 75934f3c117e..2cb49ecfbf34 100644
--- a/drivers/mmc/host/cqhci.c
+++ b/drivers/mmc/host/cqhci.c
@@ -612,6 +612,7 @@ static int cqhci_request(struct mmc_host *mmc, struct mmc_request *mrq)
 		goto out_unlock;
 	}
 
+	WARN_ON_ONCE(cq_host->slot[tag].mrq);
 	cq_host->slot[tag].mrq = mrq;
 	cq_host->slot[tag].flags = 0;
 
diff --git a/drivers/mmc/host/mmc_hsq.c b/drivers/mmc/host/mmc_hsq.c
index a5e05ed0fda3..11a4c1f3a970 100644
--- a/drivers/mmc/host/mmc_hsq.c
+++ b/drivers/mmc/host/mmc_hsq.c
@@ -227,6 +227,7 @@ static int mmc_hsq_request(struct mmc_host *mmc, struct mmc_request *mrq)
 		return -EBUSY;
 	}
 
+	WARN_ON_ONCE(hsq->slot[tag].mrq);
 	hsq->slot[tag].mrq = mrq;
 
 	/*

Thanks, 
Ming