Re: [PATCH] btrfs: drain sysfs callbacks before stopping transaction kthread

[Qu Wenruo <wqu@suse.com>]

在 2026/8/21 14:00, Jiacheng Xu 写道:
> Hi Wenruo,
> 
> I agree that rejecting sysfs writes when FS_OPEN is unset or
> CLOSING_START is set fixes the mount-time NULL pointer dereference.
> 
> However, checking these flags alone does not fully protect the teardown
> path. There is still a check-then-use race:
> 
>        sysfs store callback                close_ctree()
> 
>        test FS_OPEN == 1
>        test CLOSING_START == 0
> 
>                                             set CLOSING_START
>                                             kthread_stop(transaction_kthread)
> 
>        wake_up_process(transaction_kthread)
>        use a stopped or freed task_struct
> 
> Thus, the flag check fixes the reported initialization race, but a
> separate teardown race remains unless active sysfs callbacks are drained
> or otherwise synchronized before stopping transaction_kthread.

OK, then the next quesstion is, why we don't move the sysfs creation 
after the commit transaction creation.

Even with your patch, it didn't solve the problem that during mount the 
sysfs is created before transaction kthread.

So in theory it's possible to do sysfs write before kthread initialized, 
still causing NULL pointer dereference.

> 
> Thanks,
> Jiacheng
> 
>> -----原始邮件-----
>> 发件人: "Qu Wenruo" <wqu@suse.com>
>> 发送时间:2026-08-21 06:32:14 (星期五)
>> 收件人: "Jiacheng Xu" <stitch@zju.edu.cn>, "Chris Mason" <clm@fb.com>
>> 抄送: "David Sterba" <dsterba@suse.com>, linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org
>> 主题: Re: [PATCH] btrfs: drain sysfs callbacks before stopping transaction kthread
>>
>>
>>
>> 在 2026/8/20 21:57, Jiacheng Xu 写道:
>>> btrfs_label_store() and btrfs_feature_attr_store() wake up the
>>> transaction kthread through fs_info->transaction_kthread.
>>>
>>> During filesystem teardown, close_ctree() stops the transaction kthread
>>> before removing the mounted filesystem's sysfs attributes. A concurrent
>>> sysfs write can therefore enter one of these callbacks after the kthread
>>> has been stopped and pass an invalid task pointer to wake_up_process().
>>>
>>> This results in a concurrent null-pointer dereference in
>>> try_to_wake_up(). The scheduler is not the root cause; the invalid
>>> transaction kthread pointer is used by a Btrfs sysfs callback during
>>> teardown.
>>>
>>> Split mounted sysfs cleanup into two stages. Remove attributes which may
>>> have store callbacks before stopping the transaction kthread. The
>>> remaining sysfs kobjects are removed at the original teardown point,
>>> after the kthread has been stopped.
>>
>> Why not just simpliy reject sysfs write operations when the fs has
>> CLOSING_START or without FS_OPEN flags?
>>
>>>
>>> Apply the same ordering to the open_ctree() failure path when the
>>> transaction kthread has already been created.
>>>
>>> Tested-by: Jiacheng Xu <stitch@zju.edu.cn>
>>> Signed-off-by: Jiacheng Xu <stitch@zju.edu.cn>
>>> ---
>>> fs/btrfs/disk-io.c | 16 ++++++++++++++--
>>> fs/btrfs/sysfs.c   | 26 +++++++++++++++++++++-----
>>> fs/btrfs/sysfs.h   |  3 +++
>>> 3 files changed, 38 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>>> index 2f1666d9544e..4f5bcc576dc6 100644
>>> --- a/fs/btrfs/disk-io.c
>>> +++ b/fs/btrfs/disk-io.c
>>> @@ -3363,6 +3363,7 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
>>>         struct btrfs_root *tree_root;
>>>         struct btrfs_root *chunk_root;
>>>         struct btrfs_root *remap_root;
>>> +     bool sysfs_attrs_removed = false;
>>>         int ret;
>>>         int level;
>>>
>>> @@ -3780,6 +3781,9 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
>>> fail_qgroup:
>>>         btrfs_free_qgroup_config(fs_info);
>>> fail_trans_kthread:
>>> +     btrfs_sysfs_remove_mounted_attrs(fs_info);
>>> +     sysfs_attrs_removed = true;
>>> +
>>>         kthread_stop(fs_info->transaction_kthread);
>>>         btrfs_cleanup_transaction(fs_info);
>>>         btrfs_free_fs_roots(fs_info);
>>> @@ -3793,7 +3797,9 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
>>>         filemap_write_and_wait(fs_info->btree_inode->i_mapping);
>>>
>>> fail_sysfs:
>>> -     btrfs_sysfs_remove_mounted(fs_info);
>>> +     if (!sysfs_attrs_removed)
>>> +             btrfs_sysfs_remove_mounted_attrs(fs_info);
>>> +     btrfs_sysfs_remove_mounted_kobjects(fs_info);
>>>
>>> fail_fsdev_sysfs:
>>>         btrfs_sysfs_remove_fsid(fs_info->fs_devices);
>>> @@ -4318,6 +4324,9 @@ void __cold close_ctree(struct btrfs_fs_info *fs_info)
>>>
>>>         set_bit(BTRFS_FS_CLOSING_START, &fs_info->flags);
>>>
>>> +     /* Drain sysfs callbacks before stopping the transaction kthread. */
>>> +     btrfs_sysfs_remove_mounted_attrs(fs_info);
>>> +
>>>         /*
>>>         * If we had UNFINISHED_DROPS we could still be processing them, so
>>>         * clear that bit and wake up relocation so it can stop.
>>> @@ -4538,7 +4547,7 @@ void __cold close_ctree(struct btrfs_fs_info *fs_info)
>>>                           percpu_counter_sum(&fs_info->ordered_bytes));
>>>
>>> -     btrfs_sysfs_remove_mounted(fs_info);
>>> +     btrfs_sysfs_remove_mounted_kobjects(fs_info);
>>>         btrfs_sysfs_remove_fsid(fs_info->fs_devices);
>>>
>>>         btrfs_put_block_group_cache(fs_info);
>>>
>>> diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
>>> index 0d14570c8bc2..d90d76a152e9 100644
>>> --- a/fs/btrfs/sysfs.c
>>> +++ b/fs/btrfs/sysfs.c
>>> @@ -1707,11 +1707,23 @@ static void btrfs_sysfs_remove_fs_devices(struct btrfs_fs_devices *fs_devices)
>>>         }
>>> }
>>>
>>> -void btrfs_sysfs_remove_mounted(struct btrfs_fs_info *fs_info)
>>> +/*
>>> + * Remove attributes which may have store callbacks. kernfs waits for active
>>> + * callbacks during removal, so this must be done before stopping any kthread
>>> + * which can be woken up by those callbacks.
>>> + */
>>> +void btrfs_sysfs_remove_mounted_attrs(struct btrfs_fs_info *fs_info)
>>> {
>>>         struct kobject *fsid_kobj = &fs_info->fs_devices->fsid_kobj;
>>>
>>> -     sysfs_remove_link(fsid_kobj, "bdi");
>>> +     addrm_unknown_feature_attrs(fs_info, false);
>>> +     sysfs_remove_group(fsid_kobj, &btrfs_feature_attr_group);
>>> +     sysfs_remove_files(fsid_kobj, btrfs_attrs);
>>> +}
>>> +
>>> +static void btrfs_sysfs_remove_mounted_dirs(struct btrfs_fs_info *fs_info)
>>> +{
>>> +     sysfs_remove_link(&fs_info->fs_devices->fsid_kobj, "bdi");
>>>
>>>         if (fs_info->space_info_kobj) {
>>>               sysfs_remove_files(fs_info->space_info_kobj, allocation_attrs);
>>> @@ -1730,9 +1742,18 @@ void btrfs_sysfs_remove_mounted(struct btrfs_fs_info *fs_info)
>>>               kobject_put(fs_info->debug_kobj);
>>>         }
>>> #endif
>>> -     addrm_unknown_feature_attrs(fs_info, false);
>>> -     sysfs_remove_group(fsid_kobj, &btrfs_feature_attr_group);
>>> -     sysfs_remove_files(fsid_kobj, btrfs_attrs);
>>> +}
>>> +
>>> +void btrfs_sysfs_remove_mounted_kobjects(struct btrfs_fs_info *fs_info)
>>> +{
>>> +     btrfs_sysfs_remove_mounted_dirs(fs_info);
>>> +     btrfs_sysfs_remove_fs_devices(fs_info->fs_devices);
>>> +}
>>> +
>>> +void btrfs_sysfs_remove_mounted(struct btrfs_fs_info *fs_info)
>>> +{
>>> +     btrfs_sysfs_remove_mounted_dirs(fs_info);
>>> +     btrfs_sysfs_remove_mounted_attrs(fs_info);
>>>         btrfs_sysfs_remove_fs_devices(fs_info->fs_devices);
>>> }
>>>
>>> diff --git a/fs/btrfs/sysfs.h b/fs/btrfs/sysfs.h
>>> index 05498e5346c3..0d008fc8f1b8 100644
>>> --- a/fs/btrfs/sysfs.h
>>> +++ b/fs/btrfs/sysfs.h
>>> @@ -35,6 +35,9 @@ void btrfs_kobject_uevent(struct block_device *bdev, enum kobject_action action)
>>> int __init btrfs_init_sysfs(void);
>>> void __cold btrfs_exit_sysfs(void);
>>> int btrfs_sysfs_add_mounted(struct btrfs_fs_info *fs_info);
>>> +void btrfs_sysfs_remove_mounted_attrs(struct btrfs_fs_info *fs_info);
>>> +void btrfs_sysfs_remove_mounted_kobjects(struct btrfs_fs_info *fs_info);
>>> void btrfs_sysfs_remove_mounted(struct btrfs_fs_info *fs_info);
>>> void btrfs_sysfs_add_block_group_type(struct btrfs_block_group *cache);
>>> int btrfs_sysfs_add_space_info_type(struct btrfs_space_info *space_info);