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>

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 1bee523..b3e294e 100644 (file)
--- 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 49564ed..25e8997 100644 (file)
--- 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;