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block: kill loop_mutex

PATCH In review Petr Uzel 2 participants 2 messages Feb 25, 2011 – Mar 1, 2011 Block layer

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Petr Uzel

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#0 lore raw

PATCH 1/1 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 <[email protected]>
---
 drivers/block/loop.c |    5 -----
 1 files changed, 0 insertions(+), 5 deletions(-)
drivers/block/loop.c +0 -5
index 49e6a54..dbf31ec 100644
@@ -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;
 }
Signature · 1 line
1.7.2.3
Reviewed-by on v1 1/1
#1 lore raw
Quoted 28 lines · 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.

Quoted 7 lines

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.

Quoted 1 line
Signed-off-by Petr Uzel <[email protected]>
Reviewed-by Nikanth Karthikesan <[email protected]>
Quoted 43 lines
---
 drivers/block/loop.c |    5 -----
 1 files changed, 0 insertions(+), 5 deletions(-)
drivers/block/loop.c +0 -4
index 49e6a54..dbf31ec 100644
@@ -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:

drivers/block/loop.c +0 -1
 	mutex_unlock(&lo->lo_ctl_mutex);
 out_unlocked:
-	mutex_unlock(&loop_mutex);
 	return 0;
 }