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Following test case reproduces lockdep warning. Test case: $ mkfs.btrfs -f <dev1> $ btrfstune -S 1 <dev1> $ mount <dev1> <mnt> $ btrfs device add <dev2> <mnt> -f $ umount <mnt> $ mount <dev2> <mnt> $ umount <mnt> The warning claims a possible ABBA deadlock between the threads initiated by [#1] btrfs device add and [#0] the mount. [ 540.743122] WARNING: possible circular locking dependency detected [ 540.743129] 5.11.0-rc7+ #5 Not tainted [ 540.743135] ------------------------------------------------------ [ 540.743142] mount/2515 is trying to acquire lock: [ 540.743149] ffffa0c5544c2ce0 (&fs_devs->device_list_mutex){+.+.}-{4:4}, at: clone_fs_devices+0x6d/0x210 [btrfs] [ 540.743458] but task is already holding lock: [ 540.743461] ffffa0c54a7932b8 (btrfs-chunk-00){++++}-{4:4}, at: __btrfs_tree_read_lock+0x32/0x200 [btrfs] [ 540.743541] which lock already depends on the new lock. [ 540.743543] the existing dependency chain (in reverse order) is: [ 540.743546] -> #1 (btrfs-chunk-00){++++}-{4:4}: [ 540.743566] down_read_nested+0x48/0x2b0 [ 540.743585] __btrfs_tree_read_lock+0x32/0x200 [btrfs] [ 540.743650] btrfs_read_lock_root_node+0x70/0x200 [btrfs] [ 540.743733] btrfs_search_slot+0x6c6/0xe00 [btrfs] [ 540.743785] btrfs_update_device+0x83/0x260 [btrfs] [ 540.743849] btrfs_finish_chunk_alloc+0x13f/0x660 [btrfs] <--- device_list_mutex [ 540.743911] btrfs_create_pending_block_groups+0x18d/0x3f0 [btrfs] [ 540.743982] btrfs_commit_transaction+0x86/0x1260 [btrfs] [ 540.744037] btrfs_init_new_device+0x1600/0x1dd0 [btrfs] [ 540.744101] btrfs_ioctl+0x1c77/0x24c0 [btrfs] [ 540.744166] __x64_sys_ioctl+0xe4/0x140 [ 540.744170] do_syscall_64+0x4b/0x80 [ 540.744174] entry_SYSCALL_64_after_hwframe+0x44/0xa9 [ 540.744180] -> #0 (&fs_devs->device_list_mutex){+.+.}-{4:4}: [ 540.744184] __lock_acquire+0x155f/0x2360 [ 540.744188] lock_acquire+0x10b/0x5c0 [ 540.744190] __mutex_lock+0xb1/0xf80 [ 540.744193] mutex_lock_nested+0x27/0x30 [ 540.744196] clone_fs_devices+0x6d/0x210 [btrfs] [ 540.744270] btrfs_read_chunk_tree+0x3c7/0xbb0 [btrfs] [ 540.744336] open_ctree+0xf6e/0x2074 [btrfs] [ 540.744406] btrfs_mount_root.cold.72+0x16/0x127 [btrfs] [ 540.744472] legacy_get_tree+0x38/0x90 [ 540.744475] vfs_get_tree+0x30/0x140 [ 540.744478] fc_mount+0x16/0x60 [ 540.744482] vfs_kern_mount+0x91/0x100 [ 540.744484] btrfs_mount+0x1e6/0x670 [btrfs] [ 540.744536] legacy_get_tree+0x38/0x90 [ 540.744537] vfs_get_tree+0x30/0x140 [ 540.744539] path_mount+0x8d8/0x1070 [ 540.744541] do_mount+0x8d/0xc0 [ 540.744543] __x64_sys_mount+0x125/0x160 [ 540.744545] do_syscall_64+0x4b/0x80 [ 540.744547] entry_SYSCALL_64_after_hwframe+0x44/0xa9 [ 540.744551] other info that might help us debug this: [ 540.744552] Possible unsafe locking scenario: [ 540.744553] CPU0 CPU1 [ 540.744554] ---- ---- [ 540.744555] lock(btrfs-chunk-00); [ 540.744557] lock(&fs_devs->device_list_mutex); [ 540.744560] lock(btrfs-chunk-00); [ 540.744562] lock(&fs_devs->device_list_mutex); [ 540.744564] *** DEADLOCK *** [ 540.744565] 3 locks held by mount/2515: [ 540.744567] #0: ffffa0c56bf7a0e0 (&type->s_umount_key#42/1){+.+.}-{4:4}, at: alloc_super.isra.16+0xdf/0x450 [ 540.744574] #1: ffffffffc05a9628 (uuid_mutex){+.+.}-{4:4}, at: btrfs_read_chunk_tree+0x63/0xbb0 [btrfs] [ 540.744640] #2: ffffa0c54a7932b8 (btrfs-chunk-00){++++}-{4:4}, at: __btrfs_tree_read_lock+0x32/0x200 [btrfs] [ 540.744708] stack backtrace: [ 540.744712] CPU: 2 PID: 2515 Comm: mount Not tainted 5.11.0-rc7+ #5 But the device_list_mutex in clone_fs_devices() is redundant, as explained below. Two threads [1] and [2] (below) could lead to clone_fs_device(). [1] open_ctree <== mount sprout fs btrfs_read_chunk_tree() mutex_lock(&uuid_mutex) <== global lock read_one_dev() open_seed_devices() clone_fs_devices() <== seed fs_devices mutex_lock(&orig->device_list_mutex) <== seed fs_devices [2] btrfs_init_new_device() <== sprouting mutex_lock(&uuid_mutex); <== global lock btrfs_prepare_sprout() lockdep_assert_held(&uuid_mutex) clone_fs_devices(seed_fs_device) <== seed fs_devices Both of these threads hold uuid_mutex which is sufficient to protect getting the seed device(s) freed while we are trying to clone it for sprouting [2] or mounting a sprout [1] (as above). A mounted seed device can not free/write/replace because it is read-only. An unmounted seed device can be freed by btrfs_free_stale_devices(), but it needs uuid_mutex. So this patch removes the unnecessary device_list_mutex in clone_fs_devices(). And adds a lockdep_assert_held(&uuid_mutex) in clone_fs_devices(). Reported-by: Su Yue <l@damenly.su> Tested-by: Su Yue <l@damenly.su> Signed-off-by: Anand Jain <anand.jain@oracle.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.