Patch "mtd: fix: avoid race condition when accessing mtd->usecount" has been added to the 3.10-stable tree

This is a note to let you know that I've just added the patch titled

    mtd: fix: avoid race condition when accessing mtd->usecount

to the 3.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mtd-fix-avoid-race-condition-when-accessing-mtd-usecount.patch
and it can be found in the queue-3.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Brian Norris <[email protected]>
Date: Thu, 7 May 2015 17:55:16 -0700
Subject: mtd: fix: avoid race condition when accessing mtd->usecount

From: Brian Norris <[email protected]>

commit 073db4a51ee43ccb827f54a4261c0583b028d5ab upstream.

On A MIPS 32-cores machine a BUG_ON was triggered because some acesses to mtd->usecount were done without taking mtd_table_mutex.
kernel: Call Trace:
kernel: [<ffffffff80401818>] __put_mtd_device+0x20/0x50
kernel: [<ffffffff804086f4>] blktrans_release+0x8c/0xd8
kernel: [<ffffffff802577e0>] __blkdev_put+0x1a8/0x200
kernel: [<ffffffff802579a4>] blkdev_close+0x1c/0x30
kernel: [<ffffffff8022006c>] __fput+0xac/0x250
kernel: [<ffffffff80171208>] task_work_run+0xd8/0x120
kernel: [<ffffffff8012c23c>] work_notifysig+0x10/0x18
kernel:
kernel:

        Code: 2442ffff  ac8202d8  000217fe <00020336> dc820128  10400003
               00000000  0040f809  00000000

kernel: ---[ end trace 080fbb4579b47a73 ]---

Fixed by taking the mutex in blktrans_open and blktrans_release.

Note that this locking is already suggested in
include/linux/mtd/blktrans.h:

struct mtd_blktrans_ops {
...

	/* Called with mtd_table_mutex held; no race with add/remove */
	int (*open)(struct mtd_blktrans_dev *dev);
	void (*release)(struct mtd_blktrans_dev *dev);

...
};

But we weren't following it.

Originally reported by (and patched by) Zhang and Giuseppe, independently. Improved and rewritten.

---
 drivers/mtd/mtd_blkdevs.c |    5 +++++
 1 file changed, 5 insertions(+)

@@ -199,6 +199,7 @@ static int blktrans_open(struct block_de
 		return -ERESTARTSYS; /* FIXME: busy loop! -arnd*/
 
 	mutex_lock(&dev->lock);
+	mutex_lock(&mtd_table_mutex);
 
 	if (dev->open)
 		goto unlock;

@@ -222,6 +223,7 @@ static int blktrans_open(struct block_de
 
 unlock:
 	dev->open++;
+	mutex_unlock(&mtd_table_mutex);
 	mutex_unlock(&dev->lock);
 	blktrans_dev_put(dev);
 	return ret;

@@ -232,6 +234,7 @@ error_release:
 error_put:
 	module_put(dev->tr->owner);
 	kref_put(&dev->ref, blktrans_dev_release);
+	mutex_unlock(&mtd_table_mutex);
 	mutex_unlock(&dev->lock);
 	blktrans_dev_put(dev);
 	return ret;

@@ -245,6 +248,7 @@ static void blktrans_release(struct gend
 		return;
 
 	mutex_lock(&dev->lock);
+	mutex_lock(&mtd_table_mutex);
 
 	if (--dev->open)
 		goto unlock;

@@ -258,6 +262,7 @@ static void blktrans_release(struct gend
 		__put_mtd_device(dev->mtd);
 	}
 unlock:
+	mutex_unlock(&mtd_table_mutex);
 	mutex_unlock(&dev->lock);
 	blktrans_dev_put(dev);
 }

Patches currently in stable-queue which might be from [email protected] are

queue-3.10/mtd-fix-avoid-race-condition-when-accessing-mtd-usecount.patch queue-3.10/mtd-dc21285-use-raw-spinlock-functions-for-nw_gpio_lock.patch