[PATCH] block: Allow bdi re-registration

[Jan Kara]

SCSI can call device_add_disk() several times for one request queue when
a device in unbound and bound, creating new gendisk each time. This will
lead to bdi being repeatedly registered and unregistered. Make sure
bdi_unregister() cleans up bdi in such a way that it is prepared for
the following bdi register call.

An easy way to provoke this behavior is to enable
CONFIG_DEBUG_TEST_DRIVER_REMOVE and use scsi_debug driver to create a
scsi disk which immediately hangs without this fix.

Signed-off-by: Jan Kara <jack@xxxxxxx>
---
 mm/backing-dev.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 6d861d090e9f..6ac932210f56 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -710,6 +710,11 @@ static void cgwb_bdi_destroy(struct backing_dev_info *bdi)
 	 */
 	atomic_dec(&bdi->usage_cnt);
 	wait_event(cgwb_release_wait, !atomic_read(&bdi->usage_cnt));
+	/*
+	 * Grab back our reference so that we hold it when @bdi gets
+	 * re-registered.
+	 */
+	atomic_inc(&bdi->usage_cnt);
 }
 
 /**
@@ -857,6 +862,8 @@ int bdi_register_owner(struct backing_dev_info *bdi, struct device *owner)
 			MINOR(owner->devt));
 	if (rc)
 		return rc;
+	/* Leaking owner reference... */
+	WARN_ON(bdi->owner);
 	bdi->owner = owner;
 	get_device(owner);
 	return 0;
-- 
2.10.2


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