[PATCH] mtd_blkdevs: add mtd_table_mutex lock back to blktrans_{open, release} to avoid race condition

[Liwei Song]

without lock mtd_table_mutex in blktrans_{open, release}, there will
be a race condition when access devices /dev/mtd1 and /dev/mtdblock1
at the same time with a high frequency open and close test:

kernel BUG at drivers/mtd/mtdcore.c:1221!
lr : blktrans_release+0xb0/0x120
Call trace:
__put_mtd_device+0x4c/0x84
blktrans_release+0xb0/0x120
blkdev_put+0xd4/0x210
blkdev_close+0x34/0x50
__fput+0x8c/0x240
____fput+0x1c/0x30
task_work_run+0x98/00t_64_sync_handler+0xa4/0x130
el0t_64_sync+0x1a0/0x1a4

since the original purpose of commit 799ae31c58ae ("mtd_blkdevs:
don't hold del_mtd_blktrans_dev in blktrans_{open, release}") is
to fix a DEADLOCK issue, here convert mutex_lock to mutex_trylock
and return immediately if failed acquire mtd_table_mutex, then
both race condition and DEADLOCK can be avoided.

Fixes: 799ae31c58ae ("mtd_blkdevs: don't hold del_mtd_blktrans_dev in blktrans_{open, release}")
Signed-off-by: Liwei Song <liwei.song at windriver.com>
---
 drivers/mtd/mtd_blkdevs.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c
index b8ae1ec14e17..147e4a11dfe4 100644
--- a/drivers/mtd/mtd_blkdevs.c
+++ b/drivers/mtd/mtd_blkdevs.c
@@ -188,6 +188,8 @@ static int blktrans_open(struct block_device *bdev, fmode_t mode)
 
 	kref_get(&dev->ref);
 
+	if (!mutex_trylock(&mtd_table_mutex))
+		return ret;
 	mutex_lock(&dev->lock);
 
 	if (dev->open)
@@ -212,6 +214,7 @@ static int blktrans_open(struct block_device *bdev, fmode_t mode)
 unlock:
 	dev->open++;
 	mutex_unlock(&dev->lock);
+	mutex_unlock(&mtd_table_mutex);
 	return ret;
 
 error_release:
@@ -220,6 +223,7 @@ static int blktrans_open(struct block_device *bdev, fmode_t mode)
 error_put:
 	module_put(dev->tr->owner);
 	mutex_unlock(&dev->lock);
+	mutex_unlock(&mtd_table_mutex);
 	blktrans_dev_put(dev);
 	return ret;
 }
@@ -228,6 +232,8 @@ static void blktrans_release(struct gendisk *disk, fmode_t mode)
 {
 	struct mtd_blktrans_dev *dev = disk->private_data;
 
+	if (!mutex_trylock(&mtd_table_mutex))
+		return;
 	mutex_lock(&dev->lock);
 
 	if (--dev->open)
@@ -242,6 +248,7 @@ static void blktrans_release(struct gendisk *disk, fmode_t mode)
 	}
 unlock:
 	mutex_unlock(&dev->lock);
+	mutex_unlock(&mtd_table_mutex);
 	blktrans_dev_put(dev);
 }
 
-- 
2.32.0