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One of my CI runs popped the following lockdep splat ====================================================== WARNING: possible circular locking dependency detected 6.9.0-rc4+ #1 Not tainted ------------------------------------------------------ btrfs/471533 is trying to acquire lock: ffff92ba46980850 (&fs_info->cleaner_mutex){+.+.}-{3:3}, at: btrfs_quota_disable+0x54/0x4c0 but task is already holding lock: ffff92ba46980bd0 (&fs_info->subvol_sem){++++}-{3:3}, at: btrfs_ioctl+0x1c8f/0x2600 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #2 (&fs_info->subvol_sem){++++}-{3:3}: down_read+0x42/0x170 btrfs_rename+0x607/0xb00 btrfs_rename2+0x2e/0x70 vfs_rename+0xaf8/0xfc0 do_renameat2+0x586/0x600 __x64_sys_rename+0x43/0x50 do_syscall_64+0x95/0x180 entry_SYSCALL_64_after_hwframe+0x76/0x7e -> #1 (&sb->s_type->i_mutex_key#16){++++}-{3:3}: down_write+0x3f/0xc0 btrfs_inode_lock+0x40/0x70 prealloc_file_extent_cluster+0x1b0/0x370 relocate_file_extent_cluster+0xb2/0x720 relocate_data_extent+0x107/0x160 relocate_block_group+0x442/0x550 btrfs_relocate_block_group+0x2cb/0x4b0 btrfs_relocate_chunk+0x50/0x1b0 btrfs_balance+0x92f/0x13d0 btrfs_ioctl+0x1abf/0x2600 __x64_sys_ioctl+0x97/0xd0 do_syscall_64+0x95/0x180 entry_SYSCALL_64_after_hwframe+0x76/0x7e -> #0 (&fs_info->cleaner_mutex){+.+.}-{3:3}: __lock_acquire+0x13e7/0x2180 lock_acquire+0xcb/0x2e0 __mutex_lock+0xbe/0xc00 btrfs_quota_disable+0x54/0x4c0 btrfs_ioctl+0x206b/0x2600 __x64_sys_ioctl+0x97/0xd0 do_syscall_64+0x95/0x180 entry_SYSCALL_64_after_hwframe+0x76/0x7e other info that might help us debug this: Chain exists of: &fs_info->cleaner_mutex --> &sb->s_type->i_mutex_key#16 --> &fs_info->subvol_sem Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&fs_info->subvol_sem); lock(&sb->s_type->i_mutex_key#16); lock(&fs_info->subvol_sem); lock(&fs_info->cleaner_mutex); *** DEADLOCK *** 2 locks held by btrfs/471533: #0: ffff92ba4319e420 (sb_writers#14){.+.+}-{0:0}, at: btrfs_ioctl+0x3b5/0x2600 #1: ffff92ba46980bd0 (&fs_info->subvol_sem){++++}-{3:3}, at: btrfs_ioctl+0x1c8f/0x2600 stack backtrace: CPU: 1 PID: 471533 Comm: btrfs Kdump: loaded Not tainted 6.9.0-rc4+ #1 Call Trace: <TASK> dump_stack_lvl+0x77/0xb0 check_noncircular+0x148/0x160 ? lock_acquire+0xcb/0x2e0 __lock_acquire+0x13e7/0x2180 lock_acquire+0xcb/0x2e0 ? btrfs_quota_disable+0x54/0x4c0 ? lock_is_held_type+0x9a/0x110 __mutex_lock+0xbe/0xc00 ? btrfs_quota_disable+0x54/0x4c0 ? srso_return_thunk+0x5/0x5f ? lock_acquire+0xcb/0x2e0 ? btrfs_quota_disable+0x54/0x4c0 ? btrfs_quota_disable+0x54/0x4c0 btrfs_quota_disable+0x54/0x4c0 btrfs_ioctl+0x206b/0x2600 ? srso_return_thunk+0x5/0x5f ? __do_sys_statfs+0x61/0x70 __x64_sys_ioctl+0x97/0xd0 do_syscall_64+0x95/0x180 ? srso_return_thunk+0x5/0x5f ? reacquire_held_locks+0xd1/0x1f0 ? do_user_addr_fault+0x307/0x8a0 ? srso_return_thunk+0x5/0x5f ? lock_acquire+0xcb/0x2e0 ? srso_return_thunk+0x5/0x5f ? srso_return_thunk+0x5/0x5f ? find_held_lock+0x2b/0x80 ? srso_return_thunk+0x5/0x5f ? lock_release+0xca/0x2a0 ? srso_return_thunk+0x5/0x5f ? do_user_addr_fault+0x35c/0x8a0 ? srso_return_thunk+0x5/0x5f ? trace_hardirqs_off+0x4b/0xc0 ? srso_return_thunk+0x5/0x5f ? lockdep_hardirqs_on_prepare+0xde/0x190 ? srso_return_thunk+0x5/0x5f This happens because when we call rename we already have the inode mutex held, and then we acquire the subvol_sem if we are a subvolume. This makes the dependency inode lock -> subvol sem When we're running data relocation we will preallocate space for the data relocation inode, and we always run the relocation under the ->cleaner_mutex. This now creates the dependency of cleaner_mutex -> inode lock (from the prealloc) -> subvol_sem Qgroup delete is doing this in the opposite order, it is acquiring the subvol_sem and then it is acquiring the cleaner_mutex, which results in this lockdep splat. This deadlock can't happen in reality, because we won't ever rename the data reloc inode, nor is the data reloc inode a subvolume. However this is fairly easy to fix, simply take the cleaner mutex in the case where we are disabling qgroups before we take the subvol_sem. This resolves the lockdep splat. Reviewed-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: Josef Bacik <josef@toxicpanda.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.