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authorMing Lei <ming.lei@canonical.com>2015-05-06 12:26:23 +0800
committerJens Axboe <axboe@fb.com>2015-05-20 09:06:09 -0600
commitf8933667953e8e61bb6104f5ca88e32e85656a93 (patch)
tree25f3b20d8c1d27ac856a43f6655d9604c524fa9e /drivers
parentb816d45fd56c57180b63a03412b9f898d5736c58 (diff)
downloadlinux-f8933667953e8e61bb6104f5ca88e32e85656a93.tar.bz2
block: loop: don't hold lo_ctl_mutex in lo_open
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. Reviewed-by: Christoph Hellwig <hch@lst.de> Tested-by: Jarod Wilson <jarod@redhat.com> Acked-by: Jarod Wilson <jarod@redhat.com> Signed-off-by: Ming Lei <ming.lei@canonical.com> Signed-off-by: Jens Axboe <axboe@fb.com>
Diffstat (limited to 'drivers')
-rw-r--r--drivers/block/loop.c21
-rw-r--r--drivers/block/loop.h2
2 files changed, 13 insertions, 10 deletions
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 1bee523aa349..b3e294e529ec 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -831,7 +831,7 @@ 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;
@@ -840,6 +840,9 @@ 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;
@@ -871,6 +874,8 @@ 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;
@@ -1330,9 +1335,7 @@ 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;
@@ -1343,11 +1346,10 @@ 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)
- goto out;
+ if (atomic_dec_return(&lo->lo_refcnt))
+ return;
+ mutex_lock(&lo->lo_ctl_mutex);
if (lo->lo_flags & LO_FLAGS_AUTOCLEAR) {
/*
* In autoclear mode, stop the loop thread
@@ -1601,6 +1603,7 @@ 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;
@@ -1718,7 +1721,7 @@ 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 49564edf5581..25e8997ed246 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;