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Subject[PATCH] block: kill loop_mutex
Following steps lead to deadlock in kernel:

dd if=/dev/zero of=img bs=512 count=1000
losetup -f img
mkfs.ext2 /dev/loop0
mount -t ext2 -o loop /dev/loop0 mnt
umount mnt/

Stacktrace:
[<c102ec04>] irq_exit+0x36/0x59
[<c101502c>] smp_apic_timer_interrupt+0x6b/0x75
[<c127f639>] apic_timer_interrupt+0x31/0x38
[<c101df88>] mutex_spin_on_owner+0x54/0x5b
[<fe2250e9>] lo_release+0x12/0x67 [loop]
[<c10c4eae>] __blkdev_put+0x7c/0x10c
[<c10a4da5>] fput+0xd5/0x1aa
[<fe2250cf>] loop_clr_fd+0x1a9/0x1b1 [loop]
[<fe225110>] lo_release+0x39/0x67 [loop]
[<c10c4eae>] __blkdev_put+0x7c/0x10c
[<c10a59d9>] deactivate_locked_super+0x17/0x36
[<c10b6f37>] sys_umount+0x27e/0x2a5
[<c10b6f69>] sys_oldumount+0xb/0xe
[<c1002897>] sysenter_do_call+0x12/0x26
[<ffffffff>] 0xffffffff
Regression since 2a48fc0ab24241755dc9, which introduced the private
loop_mutex as part of the BKL removal process.

As per [1], the mutex can be safely removed.

[1] http://www.gossamer-threads.com/lists/linux/kernel/1341930

Addresses: https://bugzilla.novell.com/show_bug.cgi?id=669394
Addresses: https://bugzilla.kernel.org/show_bug.cgi?id=29172

Signed-off-by: Petr Uzel <petr.uzel@suse.cz>
---
drivers/block/loop.c | 5 -----
1 files changed, 0 insertions(+), 5 deletions(-)
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 49e6a54..dbf31ec 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -78,7 +78,6 @@

#include <asm/uaccess.h>

-static DEFINE_MUTEX(loop_mutex);
static LIST_HEAD(loop_devices);
static DEFINE_MUTEX(loop_devices_mutex);

@@ -1501,11 +1500,9 @@ static int lo_open(struct block_device *bdev, fmode_t mode)
{
struct loop_device *lo = bdev->bd_disk->private_data;

- mutex_lock(&loop_mutex);
mutex_lock(&lo->lo_ctl_mutex);
lo->lo_refcnt++;
mutex_unlock(&lo->lo_ctl_mutex);
- mutex_unlock(&loop_mutex);

return 0;
}
@@ -1515,7 +1512,6 @@ static int lo_release(struct gendisk *disk, fmode_t mode)
struct loop_device *lo = disk->private_data;
int err;

- mutex_lock(&loop_mutex);
mutex_lock(&lo->lo_ctl_mutex);

if (--lo->lo_refcnt)
@@ -1540,7 +1536,6 @@ static int lo_release(struct gendisk *disk, fmode_t mode)
out:
mutex_unlock(&lo->lo_ctl_mutex);
out_unlocked:
- mutex_unlock(&loop_mutex);
return 0;
}

--
1.7.2.3

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