Re: [PATCH v2 4/7] btrfs: update the bdev time directly when closing

[Anand Jain <anand.jain@oracle.com>]

On 28/07/2021 05:01, Josef Bacik wrote:
> We update the ctime/mtime of a block device when we remove it so that
> blkid knows the device changed.  However we do this by re-opening the
> block device and calling filp_update_time.  This is more correct because
> it'll call the inode->i_op->update_time if it exists, but the block dev
> inodes do not do this.  Instead call generic_update_time() on the
> bd_inode in order to avoid the blkdev_open path and get rid of the
> following lockdep splat
> 
> ======================================================
> WARNING: possible circular locking dependency detected
> 5.14.0-rc2+ #406 Not tainted
> ------------------------------------------------------
> losetup/11596 is trying to acquire lock:
> ffff939640d2f538 ((wq_completion)loop0){+.+.}-{0:0}, at: flush_workqueue+0x67/0x5e0
> 
> but task is already holding lock:
> ffff939655510c68 (&lo->lo_mutex){+.+.}-{3:3}, at: __loop_clr_fd+0x41/0x660 [loop]
> 
> which lock already depends on the new lock.
> 
> the existing dependency chain (in reverse order) is:
> 
> -> #4 (&lo->lo_mutex){+.+.}-{3:3}:
>         __mutex_lock+0x7d/0x750
>         lo_open+0x28/0x60 [loop]
>         blkdev_get_whole+0x25/0xf0
>         blkdev_get_by_dev.part.0+0x168/0x3c0
>         blkdev_open+0xd2/0xe0
>         do_dentry_open+0x161/0x390
>         path_openat+0x3cc/0xa20
>         do_filp_open+0x96/0x120
>         do_sys_openat2+0x7b/0x130
>         __x64_sys_openat+0x46/0x70
>         do_syscall_64+0x38/0x90
>         entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> -> #3 (&disk->open_mutex){+.+.}-{3:3}:
>         __mutex_lock+0x7d/0x750
>         blkdev_get_by_dev.part.0+0x56/0x3c0
>         blkdev_open+0xd2/0xe0
>         do_dentry_open+0x161/0x390
>         path_openat+0x3cc/0xa20
>         do_filp_open+0x96/0x120
>         file_open_name+0xc7/0x170
>         filp_open+0x2c/0x50
>         btrfs_scratch_superblocks.part.0+0x10f/0x170
>         btrfs_rm_device.cold+0xe8/0xed
>         btrfs_ioctl+0x2a31/0x2e70
>         __x64_sys_ioctl+0x80/0xb0
>         do_syscall_64+0x38/0x90
>         entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> -> #2 (sb_writers#12){.+.+}-{0:0}:
>         lo_write_bvec+0xc2/0x240 [loop]
>         loop_process_work+0x238/0xd00 [loop]
>         process_one_work+0x26b/0x560
>         worker_thread+0x55/0x3c0
>         kthread+0x140/0x160
>         ret_from_fork+0x1f/0x30
> 
> -> #1 ((work_completion)(&lo->rootcg_work)){+.+.}-{0:0}:
>         process_one_work+0x245/0x560
>         worker_thread+0x55/0x3c0
>         kthread+0x140/0x160
>         ret_from_fork+0x1f/0x30
> 
> -> #0 ((wq_completion)loop0){+.+.}-{0:0}:
>         __lock_acquire+0x10ea/0x1d90
>         lock_acquire+0xb5/0x2b0
>         flush_workqueue+0x91/0x5e0
>         drain_workqueue+0xa0/0x110
>         destroy_workqueue+0x36/0x250
>         __loop_clr_fd+0x9a/0x660 [loop]
>         block_ioctl+0x3f/0x50
>         __x64_sys_ioctl+0x80/0xb0
>         do_syscall_64+0x38/0x90
>         entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> other info that might help us debug this:
> 
> Chain exists of:
>    (wq_completion)loop0 --> &disk->open_mutex --> &lo->lo_mutex
> 
>   Possible unsafe locking scenario:
> 
>         CPU0                    CPU1
>         ----                    ----
>    lock(&lo->lo_mutex);
>                                 lock(&disk->open_mutex);
>                                 lock(&lo->lo_mutex);
>    lock((wq_completion)loop0);
> 
>   *** DEADLOCK ***
> 
> 1 lock held by losetup/11596:
>   #0: ffff939655510c68 (&lo->lo_mutex){+.+.}-{3:3}, at: __loop_clr_fd+0x41/0x660 [loop]
> 
> stack backtrace:
> CPU: 1 PID: 11596 Comm: losetup Not tainted 5.14.0-rc2+ #406
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
> Call Trace:
>   dump_stack_lvl+0x57/0x72
>   check_noncircular+0xcf/0xf0
>   ? stack_trace_save+0x3b/0x50
>   __lock_acquire+0x10ea/0x1d90
>   lock_acquire+0xb5/0x2b0
>   ? flush_workqueue+0x67/0x5e0
>   ? lockdep_init_map_type+0x47/0x220
>   flush_workqueue+0x91/0x5e0
>   ? flush_workqueue+0x67/0x5e0
>   ? verify_cpu+0xf0/0x100
>   drain_workqueue+0xa0/0x110
>   destroy_workqueue+0x36/0x250
>   __loop_clr_fd+0x9a/0x660 [loop]
>   ? blkdev_ioctl+0x8d/0x2a0
>   block_ioctl+0x3f/0x50
>   __x64_sys_ioctl+0x80/0xb0
>   do_syscall_64+0x38/0x90
>   entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>   fs/btrfs/volumes.c | 18 ++++++++++--------
>   1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index bf2449cdb2ab..3ab6c78e6eb2 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -1882,15 +1882,17 @@ static int btrfs_add_dev_item(struct btrfs_trans_handle *trans,
>    * Function to update ctime/mtime for a given device path.
>    * Mainly used for ctime/mtime based probe like libblkid.
>    */
> -static void update_dev_time(const char *path_name)
> +static void update_dev_time(struct block_device *bdev)
>   {
> -	struct file *filp;
> +	struct inode *inode = bdev->bd_inode;
> +	struct timespec64 now;
>   
> -	filp = filp_open(path_name, O_RDWR, 0);
> -	if (IS_ERR(filp))
> +	/* Shouldn't happen but just in case. */
> +	if (!inode)
>   		return;
> -	file_update_time(filp);
> -	filp_close(filp, NULL);
> +
> +	now = current_time(inode);
> +	generic_update_time(inode, &now, S_MTIME|S_CTIME);


  Oh. We could use that.

>   }
>   
>   static int btrfs_rm_dev_item(struct btrfs_device *device)
> @@ -2070,7 +2072,7 @@ void btrfs_scratch_superblocks(struct btrfs_fs_info *fs_info,
>   	btrfs_kobject_uevent(bdev, KOBJ_CHANGE);
>   
>   	/* Update ctime/mtime for device path for libblkid */
> -	update_dev_time(device_path);
> +	update_dev_time(bdev);
>   }
>   
>   int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
> @@ -2711,7 +2713,7 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
>   	btrfs_forget_devices(device_path);
>   
>   	/* Update ctime/mtime for blkid or udev */
> -	update_dev_time(device_path);
> +	update_dev_time(bdev);
>   
>   	return ret;
>   
> 

  Reviewed-by: Anand Jain <anand.jain@oracle.com>