[PATCH 3/3] [PATCH] mtd: fix concurrent access to mtd->usecount

[zhangxingcai]

__get_mtd_device() is called to increment mtd->usecount when we access
mtd via /dev/mtd1 or /dev/mtdblock1, but mtd_table_mutex lock is used in the former
via get_mtd_device(), while &dev->lock lock is used in the latter.  Therefore mtd->usecount is
not properly protected if we access /dev/mtd1 and /dev/mtdblock1 at the same time.

call graph as follows:
/dev/mtd1 --> mtdchar_open() --> get_mtd_device() --> <hold mtd_table_mutex> --> __get_mtd_device() --> <increment mtd->usecount>
/dev/mtdblock1 --> blktrans_open() --><hold &dev->lock> --> __get_mtd_device() --> <increment mtd->usecount>

Actually we triggerd the BUG_ON in put_mtd_device() on 2.6.34 kernel
due to this race.

To fix this convert mtd->usecount from int to atomic_t.

<0>------------[ cut here ]------------
<2>kernel BUG at drivers/mtd/mtdcore.c:565!
Oops: Exception in kernel mode, sig: 5 [#1]
PREEMPT SMP NR_CPUS=4 LTT NESTING LEVEL : 0
P2041 RDB
<0>last sysfs file: /sys/mbe_detect/ecc_mbe_detect
...
NIP [c037a808] put_mtd_device+0x58/0x80
LR [c037a808] put_mtd_device+0x58/0x80
Call Trace:
[ca453e90] [c037a808] put_mtd_device+0x58/0x80 (unreliable)
[ca453eb0] [c037ced8] mtd_close+0x48/0x70
[ca453ed0] [c0119078] __fput+0xe8/0x220
[ca453ef0] [c01144fc] filp_close+0x6c/0xb0
[ca453f10] [c01145fc] sys_close+0xbc/0x180
[ca453f40] [c0010ae8] ret_from_syscall+0x0/0x4

Cc: <stable at vger.kernel.org>
Signed-off-by: Zhang Xingcai <zhangxingcai at huawei.com>
---
 drivers/mtd/maps/vmu-flash.c |  2 +-
 drivers/mtd/mtdcore.c        | 12 ++++++------
 include/linux/mtd/mtd.h      |  2 +-
 3 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/drivers/mtd/maps/vmu-flash.c b/drivers/mtd/maps/vmu-flash.c
index 6b223cf..0a10779 100644
--- a/drivers/mtd/maps/vmu-flash.c
+++ b/drivers/mtd/maps/vmu-flash.c
@@ -721,7 +721,7 @@ static int vmu_can_unload(struct maple_device *mdev)
 	card = maple_get_drvdata(mdev);
 	for (x = 0; x < card->partitions; x++) {
 		mtd = &((card->mtd)[x]);
-		if (mtd->usecount > 0)
+		if (atomic_read(&mtd->usecount) > 0)
 			return 0;
 	}
 	return 1;
diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
index 4c61187..95e7cfc 100644
--- a/drivers/mtd/mtdcore.c
+++ b/drivers/mtd/mtdcore.c
@@ -402,7 +402,7 @@ int add_mtd_device(struct mtd_info *mtd)
 		goto fail_locked;
 
 	mtd->index = i;
-	mtd->usecount = 0;
+	atomic_set(&mtd->usecount, 0);
 
 	/* default value if not set by driver */
 	if (mtd->bitflip_threshold == 0)
@@ -492,9 +492,9 @@ int del_mtd_device(struct mtd_info *mtd)
 	list_for_each_entry(not, &mtd_notifiers, list)
 		not->remove(mtd);
 
-	if (mtd->usecount) {
+	if (atomic_read(&mtd->usecount)) {
 		printk(KERN_NOTICE "Removing MTD device #%d (%s) with use count %d\n",
-		       mtd->index, mtd->name, mtd->usecount);
+		       mtd->index, mtd->name, atomic_read(&mtd->usecount));
 		ret = -EBUSY;
 	} else {
 		device_unregister(&mtd->dev);
@@ -702,7 +702,7 @@ int __get_mtd_device(struct mtd_info *mtd)
 			return err;
 		}
 	}
-	mtd->usecount++;
+	atomic_inc(&mtd->usecount);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(__get_mtd_device);
@@ -756,8 +756,8 @@ EXPORT_SYMBOL_GPL(put_mtd_device);
 
 void __put_mtd_device(struct mtd_info *mtd)
 {
-	--mtd->usecount;
-	BUG_ON(mtd->usecount < 0);
+	atomic_dec(&mtd->usecount);
+	BUG_ON(atomic_read(&mtd->usecount) < 0);
 
 	if (mtd->_put_device)
 		mtd->_put_device(mtd);
diff --git a/include/linux/mtd/mtd.h b/include/linux/mtd/mtd.h
index 031ff3a..af98132 100644
--- a/include/linux/mtd/mtd.h
+++ b/include/linux/mtd/mtd.h
@@ -250,7 +250,7 @@ struct mtd_info {
 
 	struct module *owner;
 	struct device dev;
-	int usecount;
+	atomic_t usecount;
 };
 
 int mtd_erase(struct mtd_info *mtd, struct erase_info *instr);
-- 
2.1.3