[PATCH] block: kill loop_mutex

[PATCH] block: kill loop_mutex

[Petr Uzel]

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

Re: [PATCH] block: kill loop_mutex

[Nikanth Karthikesan]

On Friday, February 25, 2011 07:57:29 pm Petr Uzel wrote:
> 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
> 

BKL was recursive, but the loop_mutex is not, which results in the hang during 
umount in case of  loop over loop with LO_FLAGS_AUTOCLEAR .

And I don't see any need for loop_mutex.

> Signed-off-by: Petr Uzel <petr.uzel@suse.cz>

Reviewed-by: Nikanth Karthikesan <knikanth@suse.de>

> ---
>  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;
>  }