Re: [PATCH v3 8/8] block: Assign a lock_class per gendisk used for wait_for_completion()

Ingo Molnar
2017-10-24T10:16:01Z

* Byungchul Park <byungchul.park@lge.com> wrote:

> 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@lge.com>
> ---
> block/bio.c | 2 +-
> block/genhd.c | 13 +++++--------
> include/linux/genhd.h | 22 ++++++++++++++++++++--
> 3 files changed, 26 insertions(+), 11 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 5e901bf..cc60213 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..f195d22 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 lock_class_key *key, const char *lock_name)
> {
> struct gendisk *disk;
> struct disk_part_tbl *ptbl;
> @@ -1409,9 +1403,12 @@ 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);

lockdep_init_map() depends on CONFIG_DEBUG_LOCK_ALLOC IIRC, but the data structure
change you made depends on CONFIG_LOCKDEP_COMPLETIONS:

> 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..9832e3c 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_COMPLETIONS
> + struct lockdep_map lockdep_map;
> +#endif
> };

Which is risking a future build failure at minimum.

Isn't lockdep_map a zero size structure that is always defined? If yes then
there's no need for an #ifdef.

Also:

>
> 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, 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,
> @@ -615,6 +617,22 @@ 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,

Why is the lockdep_map passed in to the init function? Since it's wrapped in an
ugly fashion anyway, why not introduce a clean inline function that calls
lockdep_init_map() on the returned structure.

No #ifdefs required, and no uglification of the alloc_disk_node() interface.

Thanks,

Ingo