[PATCH] mmc: block: cast a error log for unbalanced mmc_blk_{open, release}

[Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>]

The intention for this patch is to help folks debug the failure
like this:

dwmmc_rockchip fe320000.dwmmc: IDMAC supports 32-bit address mode.
dwmmc_rockchip fe320000.dwmmc: Using internal DMA controller.
dwmmc_rockchip fe320000.dwmmc: Version ID is 270a
dwmmc_rockchip fe320000.dwmmc: DW MMC controller at irq 28,32 bit
host data width,256 deep fifo
dwmmc_rockchip fe320000.dwmmc: Got CD GPIO
mmc_host mmc0: Bus speed (slot 0) = 400000Hz (slot req 400000Hz, actual
400000HZ div = 0)
mmc_host mmc0: Bus speed (slot 0) = 50000000Hz (slot req 50000000Hz,
actual 50000000HZ div = 0)
mmc0: new high speed SDHC card at address 0007
mmcblk: probe of mmc0:0007 failed with error -28

It's quite annoying that some buggy userspace deamon miss the disk remove
uevent sometimes so it would finally make the SD card not work. So from
the dmesg it only shows a errno of -28 but still don't understand what
happened.

For quick reproduce this, we could set max_devices to 8 and run

for i in $(seq 1 9); do
  echo "========================" $i
  echo fe320000.dwmmc > /sys/bus/platform/drivers/dwmmc_rockchip/unbind
  sleep .5
  echo fe320000.dwmmc > /sys/bus/platform/drivers/dwmmc_rockchip/bind
  sleep .5
  mount -t vfat /dev/mmcblk0 /mnt
  sleep .5
done

Signed-off-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>
---

 drivers/mmc/core/block.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/mmc/core/block.c b/drivers/mmc/core/block.c
index a11bead..3ede32a 100644
--- a/drivers/mmc/core/block.c
+++ b/drivers/mmc/core/block.c
@@ -1985,8 +1985,24 @@ static struct mmc_blk_data *mmc_blk_alloc_req(struct mmc_card *card,
 	int devidx, ret;
 
 	devidx = ida_simple_get(&mmc_blk_ida, 0, max_devices, GFP_KERNEL);
-	if (devidx < 0)
+	if (devidx < 0) {
+		/*
+		 * The reason for why we could get -ENOSPC here for
+		 * removable devices is that probably userspace mount
+		 * the partition but forget to umount it which makes
+		 * it fail to release md->usage via mmc_blk_release,
+		 * so as a result it leaks the device ID and finally
+		 * breaks the reference counting for mmc block layer.
+		 * It's uerspace's responsibility to guarantee it but
+		 * we could cast an explcit error log  here to clarify
+		 * the situation.
+		 */
+		if (mmc_card_is_removable(card->host) && devidx == -ENOSPC)
+			dev_err(mmc_dev(card->host),
+				"possible unblanced mount/umount detected\n");
+
 		return ERR_PTR(devidx);
+	}
 
 	md = kzalloc(sizeof(struct mmc_blk_data), GFP_KERNEL);
 	if (!md) {
-- 
1.9.1


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