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Subject[PATCH v1 2/7] block: loop: don't hold lo_ctl_mutex in lo_open
Date
The lo_ctl_mutex is held for running all ioctl handlers, and
in some ioctl handlers, ioctl_by_bdev(BLKRRPART) is called for
rereading partitions, which requires bd_mutex.

So it is easy to cause failure because trylock(bd_mutex) may
fail inside blkdev_reread_part(), and follows the lock context:

blkid or other application:
->open()
->mutex_lock(bd_mutex)
->lo_open()
->mutex_lock(lo_ctl_mutex)

losetup(set fd ioctl):
->mutex_lock(lo_ctl_mutex)
->ioctl_by_bdev(BLKRRPART)
->trylock(bd_mutex)

This patch trys to eliminate the ABBA lock dependency by removing
lo_ctl_mutext in lo_open() with the following approach:

1) make lo_refcnt as atomic_t and avoid acquiring lo_ctl_mutex in lo_open():
- for open vs. add/del loop, no any problem because of loop_index_mutex
- freeze request queue during clr_fd, so I/O can't come until
clearing fd is completed, like the effect of holding lo_ctl_mutex
in lo_open
- both open() and release() have been serialized by bd_mutex already

2) don't hold lo_ctl_mutex for decreasing/checking lo_refcnt in
lo_release(), then lo_ctl_mutex is only required for the last release.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
---
drivers/block/loop.c | 19 +++++++++++--------
drivers/block/loop.h | 2 +-
2 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index c4fd1e4..0d9f014 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -881,7 +881,7 @@ static int loop_clr_fd(struct loop_device *lo)
* <dev>/do something like mkfs/losetup -d <dev> causing the losetup -d
* command to fail with EBUSY.
*/
- if (lo->lo_refcnt > 1) {
+ if (atomic_read(&lo->lo_refcnt) > 1) {
lo->lo_flags |= LO_FLAGS_AUTOCLEAR;
mutex_unlock(&lo->lo_ctl_mutex);
return 0;
@@ -890,6 +890,9 @@ static int loop_clr_fd(struct loop_device *lo)
if (filp == NULL)
return -EINVAL;

+ /* freeze request queue during the transition */
+ blk_mq_freeze_queue(lo->lo_queue);
+
spin_lock_irq(&lo->lo_lock);
lo->lo_state = Lo_rundown;
lo->lo_backing_file = NULL;
@@ -921,6 +924,8 @@ static int loop_clr_fd(struct loop_device *lo)
lo->lo_state = Lo_unbound;
/* This is safe: open() is still holding a reference. */
module_put(THIS_MODULE);
+ blk_mq_unfreeze_queue(lo->lo_queue);
+
if (lo->lo_flags & LO_FLAGS_PARTSCAN && bdev)
ioctl_by_bdev(bdev, BLKRRPART, 0);
lo->lo_flags = 0;
@@ -1378,9 +1383,7 @@ static int lo_open(struct block_device *bdev, fmode_t mode)
goto out;
}

- mutex_lock(&lo->lo_ctl_mutex);
- lo->lo_refcnt++;
- mutex_unlock(&lo->lo_ctl_mutex);
+ atomic_inc(&lo->lo_refcnt);
out:
mutex_unlock(&loop_index_mutex);
return err;
@@ -1391,11 +1394,10 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
struct loop_device *lo = disk->private_data;
int err;

- mutex_lock(&lo->lo_ctl_mutex);
-
- if (--lo->lo_refcnt)
+ if (atomic_dec_return(&lo->lo_refcnt))
goto out;

+ mutex_lock(&lo->lo_ctl_mutex);
if (lo->lo_flags & LO_FLAGS_AUTOCLEAR) {
/*
* In autoclear mode, stop the loop thread
@@ -1648,6 +1650,7 @@ static int loop_add(struct loop_device **l, int i)
disk->flags |= GENHD_FL_NO_PART_SCAN;
disk->flags |= GENHD_FL_EXT_DEVT;
mutex_init(&lo->lo_ctl_mutex);
+ atomic_set(&lo->lo_refcnt, 0);
lo->lo_number = i;
spin_lock_init(&lo->lo_lock);
disk->major = LOOP_MAJOR;
@@ -1765,7 +1768,7 @@ static long loop_control_ioctl(struct file *file, unsigned int cmd,
mutex_unlock(&lo->lo_ctl_mutex);
break;
}
- if (lo->lo_refcnt > 0) {
+ if (atomic_read(&lo->lo_refcnt) > 0) {
ret = -EBUSY;
mutex_unlock(&lo->lo_ctl_mutex);
break;
diff --git a/drivers/block/loop.h b/drivers/block/loop.h
index 301c27f..ffb6dd6 100644
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -28,7 +28,7 @@ struct loop_func_table;

struct loop_device {
int lo_number;
- int lo_refcnt;
+ atomic_t lo_refcnt;
loff_t lo_offset;
loff_t lo_sizelimit;
int lo_flags;
--
1.7.9.5


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