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SubjectRe: [PATCH] kernfs: switch global kernfs_rwsem lock to per-fs lock
Date
>This patch landed recently in linux-next (20211126) as commit 
>393c3714081a ("kernfs: switch global kernfs_rwsem lock to per-fs lock").
>In my tests I've found that it causes the following warning during the
>system reboot:
>
> =========================
> WARNING: held lock freed!
> 5.16.0-rc2+ #10984 Not tainted
> -------------------------
> kworker/1:0/18 is freeing memory ffff00004034e200-ffff00004034e3ff,
>with a lock still held there!
> ffff00004034e348 (&root->kernfs_rwsem){++++}-{3:3}, at:
>__kernfs_remove+0x310/0x37c
> 3 locks held by kworker/1:0/18:
> #0: ffff000040107938 ((wq_completion)cgroup_destroy){+.+.}-{0:0}, at:
>process_one_work+0x1f0/0x6f0
> #1: ffff80000b55bdc0
>((work_completion)(&(&css->destroy_rwork)->work)){+.+.}-{0:0}, at:
>process_one_work+0x1f0/0x6f0
> #2: ffff00004034e348 (&root->kernfs_rwsem){++++}-{3:3}, at:
>__kernfs_remove+0x310/0x37c
>
> stack backtrace:
> CPU: 1 PID: 18 Comm: kworker/1:0 Not tainted 5.16.0-rc2+ #10984
> Hardware name: Raspberry Pi 4 Model B (DT)
> Workqueue: cgroup_destroy css_free_rwork_fn
> Call trace:
> dump_backtrace+0x0/0x1ac
> show_stack+0x18/0x24
> dump_stack_lvl+0x8c/0xb8
> dump_stack+0x18/0x34
> debug_check_no_locks_freed+0x124/0x140
> kfree+0xf0/0x3a4
> kernfs_put+0x1f8/0x224
> __kernfs_remove+0x1b8/0x37c
> kernfs_destroy_root+0x38/0x50
> css_free_rwork_fn+0x288/0x3d4
> process_one_work+0x288/0x6f0
> worker_thread+0x74/0x470
> kthread+0x188/0x194
> ret_from_fork+0x10/0x20
>
>Let me know if you need more information or help in reproducing this issue.

This patch has a problem, in function kernfs_remove(), it call to
__kernfs_remove(kn), which will free the related @root of @kn, it means the
root->kernfs_rwsem will be freed too, but it is still being held by the call
to down_write(&root->kernfs_rwsem) around line number 100.

BR

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