Adding Davidlohr to CC as it's about reverting his patch.

In d5c8238849e7 ("btrfs: convert data_seqcount to seqcount_mutex_t")
the seqcount_mutex_t was added as an annotation for lockep so by revert
we'd lose that again.

On Thu, Jan 21, 2021 at 07:39:10PM +0800, Su Yue wrote:

while running xfstests on 32 bits test box, many tests failed because of
warnings in dmesg. One of those warnings(btrfs/003):
========================================================================
[   66.441305] ------------[ cut here ]------------
[   66.441317] WARNING: CPU: 6 PID: 9251 at include/linux/seqlock.h:279 btrfs_remove_chunk+0x58b/0x7b0 [btrfs]
[   66.441446] CPU: 6 PID: 9251 Comm: btrfs Tainted: G           O      5.11.0-rc4-custom+ #5
[   66.441449] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ArchLinux 1.14.0-1 04/01/2014
[   66.441451] EIP: btrfs_remove_chunk+0x58b/0x7b0 [btrfs]
[   66.441472] EAX: 00000000 EBX: 00000001 ECX: c576070c EDX: c6b15803
[   66.441475] ESI: 10000000 EDI: 00000000 EBP: c56fbcfc ESP: c56fbc70
[   66.441477] DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068 EFLAGS: 00010246
[   66.441481] CR0: 80050033 CR2: 05c8da20 CR3: 04b20000 CR4: 00350ed0
[   66.441485] Call Trace:
[   66.441510]  btrfs_relocate_chunk+0xb1/0x100 [btrfs]
[   66.441529]  ? btrfs_lookup_block_group+0x17/0x20 [btrfs]
[   66.441562]  btrfs_balance+0x8ed/0x13b0 [btrfs]
[   66.441586]  ? btrfs_ioctl_balance+0x333/0x3c0 [btrfs]
[   66.441619]  ? __this_cpu_preempt_check+0xf/0x11
[   66.441643]  btrfs_ioctl_balance+0x333/0x3c0 [btrfs]
[   66.441664]  ? btrfs_ioctl_get_supported_features+0x30/0x30 [btrfs]
[   66.441683]  btrfs_ioctl+0x414/0x2ae0 [btrfs]
[   66.441700]  ? __lock_acquire+0x35f/0x2650
[   66.441717]  ? lockdep_hardirqs_on+0x87/0x120
[   66.441720]  ? lockdep_hardirqs_on_prepare+0xd0/0x1e0
[   66.441724]  ? call_rcu+0x2d3/0x530
[   66.441731]  ? __might_fault+0x41/0x90
[   66.441736]  ? kvm_sched_clock_read+0x15/0x50
[   66.441740]  ? sched_clock+0x8/0x10
[   66.441745]  ? sched_clock_cpu+0x13/0x180
[   66.441750]  ? btrfs_ioctl_get_supported_features+0x30/0x30 [btrfs]
[   66.441750]  ? btrfs_ioctl_get_supported_features+0x30/0x30 [btrfs]
[   66.441768]  __ia32_sys_ioctl+0x165/0x8a0
[   66.441773]  ? __this_cpu_preempt_check+0xf/0x11
[   66.441785]  ? __might_fault+0x89/0x90
[   66.441791]  __do_fast_syscall_32+0x54/0x80
[   66.441796]  do_fast_syscall_32+0x32/0x70
[   66.441801]  do_SYSENTER_32+0x15/0x20
[   66.441805]  entry_SYSENTER_32+0x9f/0xf2
[   66.441808] EIP: 0xab7b5549
[   66.441814] EAX: ffffffda EBX: 00000003 ECX: c4009420 EDX: bfa91f5c
[   66.441816] ESI: 00000003 EDI: 00000001 EBP: 00000000 ESP: bfa91e98
[   66.441818] DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 007b EFLAGS: 00000292
[   66.441833] irq event stamp: 42579
[   66.441835] hardirqs last  enabled at (42585): [<c60eb065>] console_unlock+0x495/0x590
[   66.441838] hardirqs last disabled at (42590): [<c60eafd5>] console_unlock+0x405/0x590
[   66.441840] softirqs last  enabled at (41698): [<c601b76c>] call_on_stack+0x1c/0x60
[   66.441843] softirqs last disabled at (41681): [<c601b76c>] call_on_stack+0x1c/0x60
[   66.441846] ---[ end trace 7a9311b3f90a392e ]---
========================================================================

========================================================================
btrfs_remove_chunk+0x58b/0x7b0:
__seqprop_mutex_assert at linux/./include/linux/seqlock.h:279
(inlined by) btrfs_device_set_bytes_used at linux/fs/btrfs/volumes.h:212
(inlined by) btrfs_remove_chunk at linux/fs/btrfs/volumes.c:2994
========================================================================

The warning is produced by lockdep_assert_held() in
__seqprop_mutex_assert() if CONFIG_LOCKDEP is enabled.
And "volumes.c:2994" is btrfs_device_set_bytes_used() with mutex lock
&fs_info->chunk_mutex hold already.

After adding some debug prints, the cause was found that manyq
__alloc_device() are called with NULL @fs_info. Inside the function,
btrfs_device_data_ordered_init() is expanded to seqcount_mutex_init().
In this scenario, its second parameter(&info->chunk_mutex) passed is
&NULL->chunk_mutex which equals to offsetof(struct btrfs_fs_info,
chunk_mutex) unexpectly. Thus, seqcount_mutex_init() is called in wrong
way. And later btrfs_get/set_device_*() helpers triger lockdep warnings.

The complex solution is to delay the call of btrfs_device_data_ordered_
init() so the lockdep functionality can work well.
It requires that no btrfs_get/set_device_*() is called between
btrfs_alloc_device with NULL @fs_info and the delayed
btrfs_device_data_ordered_init(). Otherwise, total_bytes, disk_total_
bytes and bytes_uesed of device may be inconsistent in 32 bits
environments.

If the fs_info is not available at all times, would it be possible to
set it once it is? (And reset when the fs_info is released).