[PATCH] mmc: allow mmc to block wait_for_device_probe()

[Dennis Zhou <dennis@kernel.org>]

I've been hitting a failed data device lookup when using dm-verity and a
root device on an emmc partition. This is because there is a race where
dm-verity is looking for a data device, but the partitions on the emmc
device haven't been probed yet.

Initially I looked at solving this by changing devt_from_devname() to
look for partitions, but it seems there is legacy reasons and issues due
to dm.

MMC uses 2 levels of probing. The first to handle initializing the
host and the second to iterate attached devices. The second is done by
a workqueue item. However, this paradigm makes wait_for_device_probe()
useless as a barrier for when we can assume attached devices have been
probed.

This patch fixes this by exposing 2 methods inc/dec_probe_count() to
allow device drivers that do asynchronous probing to delay waiters on
wait_for_device_probe() so that when they are released, they can assume
attached devices have been probed.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
---
 drivers/base/dd.c        | 17 +++++++++++++++--
 drivers/mmc/core/core.c  | 25 +++++++++++++++++++++++--
 include/linux/device.h   |  7 +++++++
 include/linux/mmc/host.h |  1 +
 4 files changed, 46 insertions(+), 4 deletions(-)

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 95ae347df137..c0117476e1d6 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -494,6 +494,19 @@ EXPORT_SYMBOL_GPL(device_bind_driver);
 static atomic_t probe_count = ATOMIC_INIT(0);
 static DECLARE_WAIT_QUEUE_HEAD(probe_waitqueue);
 
+void inc_probe_count(void)
+{
+	atomic_inc(&probe_count);
+}
+EXPORT_SYMBOL_GPL(inc_probe_count);
+
+void dec_probe_count(void)
+{
+	if (atomic_dec_return(&probe_count) == 0)
+		wake_up_all(&probe_waitqueue);
+}
+EXPORT_SYMBOL_GPL(dec_probe_count);
+
 static ssize_t state_synced_show(struct device *dev,
 				 struct device_attribute *attr, char *buf)
 {
@@ -793,8 +806,8 @@ static int driver_probe_device(struct device_driver *drv, struct device *dev)
 		    !defer_all_probes)
 			driver_deferred_probe_trigger();
 	}
-	atomic_dec(&probe_count);
-	wake_up_all(&probe_waitqueue);
+	if (atomic_dec_return(&probe_count) == 0)
+		wake_up_all(&probe_waitqueue);
 	return ret;
 }
 
diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
index 368f10405e13..92690984dac2 100644
--- a/drivers/mmc/core/core.c
+++ b/drivers/mmc/core/core.c
@@ -2192,11 +2192,11 @@ void mmc_rescan(struct work_struct *work)
 	int i;
 
 	if (host->rescan_disable)
-		return;
+		goto out_probe;
 
 	/* If there is a non-removable card registered, only scan once */
 	if (!mmc_card_is_removable(host) && host->rescan_entered)
-		return;
+		goto out_probe;
 	host->rescan_entered = 1;
 
 	if (host->trigger_card_event && host->ops->card_event) {
@@ -2247,6 +2247,13 @@ void mmc_rescan(struct work_struct *work)
  out:
 	if (host->caps & MMC_CAP_NEEDS_POLL)
 		mmc_schedule_delayed_work(&host->detect, HZ);
+
+out_probe:
+	if (host->start_probe) {
+		/* matches inc_probe_count() in mmc_start_host() */
+		dec_probe_count();
+		host->start_probe = 0;
+	}
 }
 
 void mmc_start_host(struct mmc_host *host)
@@ -2261,6 +2268,15 @@ void mmc_start_host(struct mmc_host *host)
 	}
 
 	mmc_gpiod_request_cd_irq(host);
+
+	/*
+	 * MMC uses 2 levels of probing. The first to handle initializing the
+	 * host and the second to iterate attached devices. However, this
+	 * paradigm breaks wait_for_device_probe(). Fix this here by
+	 * incrementing the probe_count and decrementing after the scan.
+	 */
+	host->start_probe = 1;
+	inc_probe_count();
 	_mmc_detect_change(host, 0, false);
 }
 
@@ -2273,6 +2289,11 @@ void __mmc_stop_host(struct mmc_host *host)
 
 	host->rescan_disable = 1;
 	cancel_delayed_work_sync(&host->detect);
+	/* start_probe is protected by the cancel_delayed_work_sync() */
+	if (host->start_probe) {
+		dec_probe_count();
+		host->start_probe = 0;
+	}
 }
 
 void mmc_stop_host(struct mmc_host *host)
diff --git a/include/linux/device.h b/include/linux/device.h
index e270cb740b9e..d09bdc33d1cf 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -891,6 +891,13 @@ int __must_check device_reprobe(struct device *dev);
 
 bool device_is_bound(struct device *dev);
 
+/*
+ * Functions that inc/dec probe_count to allow device drivers that finish
+ * probing asynchronously to delay wait_for_device_probe() appropriately.
+ */
+void inc_probe_count(void);
+void dec_probe_count(void);
+
 /*
  * Easy functions for dynamically creating devices on the fly
  */
diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
index 0c0c9a0fdf57..ea7b9158f052 100644
--- a/include/linux/mmc/host.h
+++ b/include/linux/mmc/host.h
@@ -428,6 +428,7 @@ struct mmc_host {
 	unsigned int		retune_paused:1; /* re-tuning is temporarily disabled */
 	unsigned int		retune_crc_disable:1; /* don't trigger retune upon crc */
 	unsigned int		can_dma_map_merge:1; /* merging can be used */
+	unsigned int		start_probe:1; /* if this is our first scan */
 
 	int			rescan_disable;	/* disable card detection */
 	int			rescan_entered;	/* used with nonremovable devices */
-- 
2.40.0