Darrick posted the following warning and Dave Chinner analyzed it:

quoted

======================================================
WARNING: possible circular locking dependency detected
4.14.0-rc1-fixes #1 Tainted: G        W
------------------------------------------------------
loop0/31693 is trying to acquire lock:
 (&(&ip->i_mmaplock)->mr_lock){++++}, at: [<ffffffffa00f1b0c>] xfs_ilock+0x23c/0x330 [xfs]

but now in release context of a crosslock acquired at the following:
 ((complete)&ret.event){+.+.}, at: [<ffffffff81326c1f>] submit_bio_wait+0x7f/0xb0

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 ((complete)&ret.event){+.+.}:
       lock_acquire+0xab/0x200
       wait_for_completion_io+0x4e/0x1a0
       submit_bio_wait+0x7f/0xb0
       blkdev_issue_zeroout+0x71/0xa0
       xfs_bmapi_convert_unwritten+0x11f/0x1d0 [xfs]
       xfs_bmapi_write+0x374/0x11f0 [xfs]
       xfs_iomap_write_direct+0x2ac/0x430 [xfs]
       xfs_file_iomap_begin+0x20d/0xd50 [xfs]
       iomap_apply+0x43/0xe0
       dax_iomap_rw+0x89/0xf0
       xfs_file_dax_write+0xcc/0x220 [xfs]
       xfs_file_write_iter+0xf0/0x130 [xfs]
       __vfs_write+0xd9/0x150
       vfs_write+0xc8/0x1c0
       SyS_write+0x45/0xa0
       entry_SYSCALL_64_fastpath+0x1f/0xbe

-> #1 (&xfs_nondir_ilock_class){++++}:
       lock_acquire+0xab/0x200
       down_write_nested+0x4a/0xb0
       xfs_ilock+0x263/0x330 [xfs]
       xfs_setattr_size+0x152/0x370 [xfs]
       xfs_vn_setattr+0x6b/0x90 [xfs]
       notify_change+0x27d/0x3f0
       do_truncate+0x5b/0x90
       path_openat+0x237/0xa90
       do_filp_open+0x8a/0xf0
       do_sys_open+0x11c/0x1f0
       entry_SYSCALL_64_fastpath+0x1f/0xbe

-> #0 (&(&ip->i_mmaplock)->mr_lock){++++}:
       up_write+0x1c/0x40
       xfs_iunlock+0x1d0/0x310 [xfs]
       xfs_file_fallocate+0x8a/0x310 [xfs]
       loop_queue_work+0xb7/0x8d0
       kthread_worker_fn+0xb9/0x1f0

Chain exists of:
  &(&ip->i_mmaplock)->mr_lock --> &xfs_nondir_ilock_class --> (complete)&ret.event

 Possible unsafe locking scenario by crosslock:

       CPU0                    CPU1
       ----                    ----
  lock(&xfs_nondir_ilock_class);
  lock((complete)&ret.event);
                               lock(&(&ip->i_mmaplock)->mr_lock);
                               unlock((complete)&ret.event);

               *** DEADLOCK ***

The warning is a false positive, caused by the fact that all
wait_for_completion()s in submit_bio_wait() are waiting with the same
lock class.

However, some bios have nothing to do with others, for example, the case
might happen while using loop devices, between bios of an upper device
and a lower device(=loop device).

The safest way to assign different lock classes to different devices is
to do it for each gendisk. In other words, this patch assigns a
lockdep_map per gendisk and uses it when initializing completion in
submit_bio_wait().

Of course, it might be too conservative. But, making it safest for now
and extended by block layer experts later is good, at the moment.

Reported-by: Darrick J. Wong <redacted>
Analyzed-by: Dave Chinner [off-list ref]
Signed-off-by: Byungchul Park <redacted>
---
 block/bio.c           |  2 +-
 block/genhd.c         | 10 ++--------
 include/linux/genhd.h | 24 ++++++++++++++++++++++--
 3 files changed, 25 insertions(+), 11 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 99d0ca5..a3cb1d1 100644
--- a/block/bio.c
+++ b/block/bio.c

@@ -935,7 +935,7 @@ static void submit_bio_wait_endio(struct bio *bio)
  */
 int submit_bio_wait(struct bio *bio)
 {
-	DECLARE_COMPLETION_ONSTACK(done);
+	DECLARE_COMPLETION_ONSTACK_MAP(done, bio->bi_disk->lockdep_map);
 
 	bio->bi_private = &done;
 	bio->bi_end_io = submit_bio_wait_endio;

diff --git a/block/genhd.c b/block/genhd.c
index dd305c6..630c0da 100644
--- a/block/genhd.c
+++ b/block/genhd.c

@@ -1354,13 +1354,7 @@ dev_t blk_lookup_devt(const char *name, int partno)
 }
 EXPORT_SYMBOL(blk_lookup_devt);
 
-struct gendisk *alloc_disk(int minors)
-{
-	return alloc_disk_node(minors, NUMA_NO_NODE);
-}
-EXPORT_SYMBOL(alloc_disk);
-
-struct gendisk *alloc_disk_node(int minors, int node_id)
+struct gendisk *__alloc_disk_node(int minors, int node_id)
 {
 	struct gendisk *disk;
 	struct disk_part_tbl *ptbl;

@@ -1411,7 +1405,7 @@ struct gendisk *alloc_disk_node(int minors, int node_id)
 	}
 	return disk;
 }
-EXPORT_SYMBOL(alloc_disk_node);
+EXPORT_SYMBOL(__alloc_disk_node);
 
 struct kobject *get_disk(struct gendisk *disk)
 {

diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index 6d85a75..f6ec6a2 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h

@@ -206,6 +206,9 @@ struct gendisk {
 #endif	/* CONFIG_BLK_DEV_INTEGRITY */
 	int node_id;
 	struct badblocks *bb;
+#ifdef CONFIG_LOCKDEP
+	struct lockdep_map lockdep_map;
+#endif
 };
 
 static inline struct gendisk *part_to_disk(struct hd_struct *part)

@@ -590,8 +593,7 @@ extern struct hd_struct * __must_check add_partition(struct gendisk *disk,
 extern void delete_partition(struct gendisk *, int);
 extern void printk_all_partitions(void);
 
-extern struct gendisk *alloc_disk_node(int minors, int node_id);
-extern struct gendisk *alloc_disk(int minors);
+extern struct gendisk *__alloc_disk_node(int minors, int node_id);
 extern struct kobject *get_disk(struct gendisk *disk);
 extern void put_disk(struct gendisk *disk);
 extern void blk_register_region(dev_t devt, unsigned long range,

@@ -615,6 +617,24 @@ extern ssize_t part_fail_store(struct device *dev,
 			       const char *buf, size_t count);
 #endif /* CONFIG_FAIL_MAKE_REQUEST */
 
+#define alloc_disk_node(m, id) \
+({									\
+	static struct lock_class_key __key;				\
+	const char *__name;						\
+	struct gendisk *ret;						\
+									\
+	__name = "(complete)"#m"("#id")";				\
+									\
+	ret = __alloc_disk_node(m, id);					\
+									\
+	if (ret)							\
+		lockdep_init_map(&ret->lockdep_map, __name, &__key, 0); \
+									\
+	ret;								\
+})