[RFC] lockdep: Assign a lock_class per gendisk used for wait_for_completion()

From: Byungchul Park
Date: Mon Oct 16 2017 - 05:51:51 EST

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Darrick and Dave Chinner posted the following warning:

> ======================================================
> 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, atm.

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
block/bio.c | 4 ++--
block/genhd.c | 13 +++++--------
include/linux/genhd.h | 26 ++++++++++++++++++++++----
3 files changed, 29 insertions(+), 14 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 9a63597..0d4d6c0 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -941,7 +941,7 @@ int submit_bio_wait(struct bio *bio)
{
struct submit_bio_ret ret;

- init_completion(&ret.event);
+ init_completion_with_map(&ret.event, &bio->bi_disk->lockdep_map);
bio->bi_private = &ret;
bio->bi_end_io = submit_bio_wait_endio;
bio->bi_opf |= REQ_SYNC;
@@ -1382,7 +1382,7 @@ struct bio *bio_map_user_iov(struct request_queue *q,

if (len <= 0)
break;
-
+
if (bytes > len)
bytes = len;

diff --git a/block/genhd.c b/block/genhd.c
index 7f520fa..676c245 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1304,13 +1304,7 @@ 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 lock_class_key *key, const char *lock_name)
{
struct gendisk *disk;

@@ -1350,9 +1344,12 @@ struct gendisk *alloc_disk_node(int minors, int node_id)
disk_to_dev(disk)->type = &disk_type;
device_initialize(disk_to_dev(disk));
}
+
+ lockdep_init_map(&disk->lockdep_map, lock_name, key, 0);
+
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 e619fae..5225efc 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -3,7 +3,7 @@

/*
* genhd.h Copyright (C) 1992 Drew Eckhardt
- * Generic hard disk header file by
+ * Generic hard disk header file by
* Drew Eckhardt
*
* <drew@xxxxxxxxxxxx>
@@ -206,6 +206,9 @@ struct gendisk {
#endif /* CONFIG_BLK_DEV_INTEGRITY */
int node_id;
struct badblocks *bb;
+#ifdef CONFIG_LOCKDEP_COMPLETIONS
+ struct lockdep_map lockdep_map;
+#endif
};

static inline struct gendisk *part_to_disk(struct hd_struct *part)
@@ -483,7 +486,7 @@ struct bsd_disklabel {
__s16 d_type; /* drive type */
__s16 d_subtype; /* controller/d_type specific */
char d_typename[16]; /* type name, e.g. "eagle" */
- char d_packname[16]; /* pack identifier */
+ char d_packname[16]; /* pack identifier */
__u32 d_secsize; /* # of bytes per sector */
__u32 d_nsectors; /* # of data sectors per track */
__u32 d_ntracks; /* # of tracks per cylinder */
@@ -602,8 +605,7 @@ 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, struct lock_class_key *key, const char *lock_name);
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,
@@ -627,6 +629,22 @@ extern ssize_t part_fail_store(struct device *dev,
const char *buf, size_t count);
#endif /* CONFIG_FAIL_MAKE_REQUEST */

+#ifdef CONFIG_LOCKDEP_COMPLETIONS
+#define alloc_disk_node(m, id) \
+({ \
+ static struct lock_class_key __key; \
+ const char *__lock_name; \
+ \
+ __lock_name = "(complete)"#m"("#id")"; \
+ \
+ __alloc_disk_node(m, id, &__key, __lock_name); \
+})
+#else
+#define alloc_disk_node(m, id) __alloc_disk_node(m, id, NULL, NULL)
+#endif
+
+#define alloc_disk(m) alloc_disk_node(m, NUMA_NO_NODE)
+
static inline int hd_ref_init(struct hd_struct *part)
{
if (percpu_ref_init(&part->ref, __delete_partition, 0,
--
1.9.1

Next message: Li, Aubrey: "Re: [RFC PATCH v2 3/8] cpuidle: add a new predict interface"
Previous message: Peter Maydell: "Re: [PATCH 7/9] KVM: arm/arm64: vgic-its: free caches when GITS_BASER Valid bit is cleared"
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