On Wed, Jun 21, 2017 at 02:21:12PM -0700, Tahsin Erdogan wrote:
> Setting a large xattr value may require writing the attribute contents
> to an external inode. In this case we may need to lock the xattr inode
> along with the parent inode. This doesn't pose a deadlock risk because
> xattr inodes are not directly visible to the user and their access is
> restricted.
>
> Assign a lockdep subclass to xattr inode's lock.
>
> ============================================
> WARNING: possible recursive locking detected
> 4.12.0-rc1+ #740 Not tainted
> --------------------------------------------
> python/1822 is trying to acquire lock:
> (&sb->s_type->i_mutex_key#15){+.+...}, at: [<ffffffff804912ca>] ext4_xattr_set_entry+0x65a/0x7b0
>
> but task is already holding lock:
> (&sb->s_type->i_mutex_key#15){+.+...}, at: [<ffffffff803d6687>] vfs_setxattr+0x57/0xb0
>
> other info that might help us debug this:
> Possible unsafe locking scenario:
>
> CPU0
> ----
> lock(&sb->s_type->i_mutex_key#15);
> lock(&sb->s_type->i_mutex_key#15);
>
> *** DEADLOCK ***
>
> May be due to missing lock nesting notation
>
> 4 locks held by python/1822:
> #0: (sb_writers#10){.+.+.+}, at: [<ffffffff803d0eef>] mnt_want_write+0x1f/0x50
> #1: (&sb->s_type->i_mutex_key#15){+.+...}, at: [<ffffffff803d6687>] vfs_setxattr+0x57/0xb0
> #2: (jbd2_handle){.+.+..}, at: [<ffffffff80493f40>] start_this_handle+0xf0/0x420
> #3: (&ei->xattr_sem){++++..}, at: [<ffffffff804920ba>] ext4_xattr_set_handle+0x9a/0x4f0
>
> stack backtrace:
> CPU: 0 PID: 1822 Comm: python Not tainted 4.12.0-rc1+ #740
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> Call Trace:
> dump_stack+0x67/0x9e
> __lock_acquire+0x5f3/0x1750
> lock_acquire+0xb5/0x1d0
> down_write+0x2c/0x60
> ext4_xattr_set_entry+0x65a/0x7b0
> ext4_xattr_block_set+0x1b2/0x9b0
> ext4_xattr_set_handle+0x322/0x4f0
> ext4_xattr_set+0x144/0x1a0
> ext4_xattr_user_set+0x34/0x40
> __vfs_setxattr+0x66/0x80
> __vfs_setxattr_noperm+0x69/0x1c0
> vfs_setxattr+0xa2/0xb0
> setxattr+0x12e/0x150
> path_setxattr+0x87/0xb0
> SyS_setxattr+0xf/0x20
> entry_SYSCALL_64_fastpath+0x18/0xad
>
> Signed-off-by: Tahsin Erdogan <tahsin@xxxxxxxxxx>
Thanks, applied.
- Ted
