On 18/7/20 3:12 am, Josef Bacik wrote:
We are currently getting this lockdep splat
======================================================
WARNING: possible circular locking dependency detected
5.8.0-rc5+ #20 Tainted: G E
------------------------------------------------------
mount/678048 is trying to acquire lock:
ffff9b769f15b6e0 (&fs_devs->device_list_mutex){+.+.}-{3:3}, at: clone_fs_devices+0x4d/0x170 [btrfs]
but task is already holding lock:
ffff9b76abdb08d0 (&fs_info->chunk_mutex){+.+.}-{3:3}, at: btrfs_read_chunk_tree+0x6a/0x800 [btrfs]
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #1 (&fs_info->chunk_mutex){+.+.}-{3:3}:
__mutex_lock+0x8b/0x8f0
btrfs_init_new_device+0x2d2/0x1240 [btrfs]
btrfs_ioctl+0x1de/0x2d20 [btrfs]
ksys_ioctl+0x87/0xc0
__x64_sys_ioctl+0x16/0x20
do_syscall_64+0x52/0xb0
entry_SYSCALL_64_after_hwframe+0x44/0xa9
-> #0 (&fs_devs->device_list_mutex){+.+.}-{3:3}:
__lock_acquire+0x1240/0x2460
lock_acquire+0xab/0x360
__mutex_lock+0x8b/0x8f0
clone_fs_devices+0x4d/0x170 [btrfs]
btrfs_read_chunk_tree+0x330/0x800 [btrfs]
open_ctree+0xb7c/0x18ce [btrfs]
btrfs_mount_root.cold+0x13/0xfa [btrfs]
legacy_get_tree+0x30/0x50
vfs_get_tree+0x28/0xc0
fc_mount+0xe/0x40
vfs_kern_mount.part.0+0x71/0x90
btrfs_mount+0x13b/0x3e0 [btrfs]
legacy_get_tree+0x30/0x50
vfs_get_tree+0x28/0xc0
do_mount+0x7de/0xb30
__x64_sys_mount+0x8e/0xd0
do_syscall_64+0x52/0xb0
entry_SYSCALL_64_after_hwframe+0x44/0xa9
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&fs_info->chunk_mutex);
lock(&fs_devs->device_list_mutex);
lock(&fs_info->chunk_mutex);
lock(&fs_devs->device_list_mutex);
*** DEADLOCK ***
3 locks held by mount/678048:
#0: ffff9b75ff5fb0e0 (&type->s_umount_key#63/1){+.+.}-{3:3}, at: alloc_super+0xb5/0x380
#1: ffffffffc0c2fbc8 (uuid_mutex){+.+.}-{3:3}, at: btrfs_read_chunk_tree+0x54/0x800 [btrfs]
#2: ffff9b76abdb08d0 (&fs_info->chunk_mutex){+.+.}-{3:3}, at: btrfs_read_chunk_tree+0x6a/0x800 [btrfs]
stack backtrace:
CPU: 2 PID: 678048 Comm: mount Tainted: G E 5.8.0-rc5+ #20
Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./890FX Deluxe5, BIOS P1.40 05/03/2011
Call Trace:
dump_stack+0x96/0xd0
check_noncircular+0x162/0x180
__lock_acquire+0x1240/0x2460
? asm_sysvec_apic_timer_interrupt+0x12/0x20
lock_acquire+0xab/0x360
? clone_fs_devices+0x4d/0x170 [btrfs]
__mutex_lock+0x8b/0x8f0
? clone_fs_devices+0x4d/0x170 [btrfs]
? rcu_read_lock_sched_held+0x52/0x60
? cpumask_next+0x16/0x20
? module_assert_mutex_or_preempt+0x14/0x40
? __module_address+0x28/0xf0
? clone_fs_devices+0x4d/0x170 [btrfs]
? static_obj+0x4f/0x60
? lockdep_init_map_waits+0x43/0x200
? clone_fs_devices+0x4d/0x170 [btrfs]
clone_fs_devices+0x4d/0x170 [btrfs]
btrfs_read_chunk_tree+0x330/0x800 [btrfs]
open_ctree+0xb7c/0x18ce [btrfs]
? super_setup_bdi_name+0x79/0xd0
btrfs_mount_root.cold+0x13/0xfa [btrfs]
? vfs_parse_fs_string+0x84/0xb0
? rcu_read_lock_sched_held+0x52/0x60
? kfree+0x2b5/0x310
legacy_get_tree+0x30/0x50
vfs_get_tree+0x28/0xc0
fc_mount+0xe/0x40
vfs_kern_mount.part.0+0x71/0x90
btrfs_mount+0x13b/0x3e0 [btrfs]
? cred_has_capability+0x7c/0x120
? rcu_read_lock_sched_held+0x52/0x60
? legacy_get_tree+0x30/0x50
legacy_get_tree+0x30/0x50
vfs_get_tree+0x28/0xc0
do_mount+0x7de/0xb30
? memdup_user+0x4e/0x90
__x64_sys_mount+0x8e/0xd0
do_syscall_64+0x52/0xb0
entry_SYSCALL_64_after_hwframe+0x44/0xa9
This is because btrfs_read_chunk_tree() can come upon DEV_EXTENT's and
then read the device, which takes the device_list_mutex. The
device_list_mutex needs to be taken before the chunk_mutex, so this is a
problem. We only really need the chunk mutex around adding the chunk,
so move the mutex around read_one_chunk.
An argument could be made that we don't even need the chunk_mutex here
as it's during mount, and we are protected by various other locks.
However we already have special rules for ->device_list_mutex, and I'd
rather not have another special case for ->chunk_mutex.
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
Reviewed-by: Anand Jain <anand.jain@xxxxxxxxxx>
Thanks, Anand
