Re: [PATCH RFC 1/1] kernfs: keep kernfs node alive for __kernfs_remove()

[Tejun Heo <tj@kernel.org>]

On Tue, Apr 16, 2019 at 06:53:35PM +0300, Konstantin Khorenko wrote:
> __kernfs_remove() which is called under kernfs_mutex,
> assumes nobody kills kernfs node whie it's working on it
> and "get"s current kernfs node for that.
> 
> But we hit a warning in kernfs_get(): kn->counter == 0 already:
>   ------------[ cut here ]------------
>   WARNING: CPU: 2 PID: 63923 at fs/kernfs/dir.c:377 kernfs_get+0x2f/0x40
>   ...
>   Call Trace:
>    [<ffffffffa7f92e67>] dump_stack+0x19/0x1b
>    [<ffffffffa78987b8>] __warn+0xd8/0x100
>    [<ffffffffa78988fd>] warn_slowpath_null+0x1d/0x20
>    [<ffffffffa7aecaff>] kernfs_get+0x2f/0x40
>    [<ffffffffa7aed233>] __kernfs_remove+0x113/0x260
>    [<ffffffffa7aee201>] kernfs_remove+0x21/0x30
>    [<ffffffffa7af1010>] sysfs_remove_dir+0x50/0x80
>    [<ffffffffa7b9fb38>] kobject_del+0x18/0x50
>    [<ffffffffa7a38a4d>] sysfs_slab_remove+0x3d/0x50
>    [<ffffffffa79f1e6b>] do_kmem_cache_release+0x3b/0x70
>    [<ffffffffa79f2aa1>] memcg_destroy_kmem_caches+0xb1/0xf0
>    [<ffffffffa7a4ed5c>] mem_cgroup_css_free+0x4c/0x280
>    [<ffffffffa79377fc>] cgroup_free_fn+0x4c/0x120
>    [<ffffffffa78bc222>] process_one_work+0x182/0x440
>    [<ffffffffa78bd3d6>] worker_thread+0x126/0x3c0
>    [<ffffffffa78c4441>] kthread+0xd1/0xe0
> 
> This could be for example because of kernfs_notify_workfn() which
> does kernfs_put(kn) out of kernfs_mutex held section,
> so move kernfs_put(kn) under the mutex.

This patch doesn't really make sense to me.  Can you give a more
concrete scenario where this would help?

Thanks.

-- 
tejun