linux-kernel - misc_deregister() throwing warning in ida

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Message-ID: <CALkUMdT_Q9o-NDKhAk=v_ARSe541y6yeg8hacYJ2iZ5PGjjRVw@mail.gmail.com>
Date: Mon, 16 Sep 2024 18:56:18 +0530
From: Vimal Agrawal <avimalin@...il.com>
To: linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	D Scott Phillips <scott@...amperecomputing.com>, Vimal Agrawal <vimal.agrawal@...hos.com>
Subject: misc_deregister() throwing warning in ida_free()

Hi Scott/ Greg and all,

We recently upgraded kernel from 6.1 to 6.6.49 and started seeing
following WARNING during misc_deregister():

------------[ cut here ]------------

WARNING: CPU: 0 PID: 159 at lib/idr.c:525 ida_free+0x3e0/0x41f
ida_free called for id=127 which is not allocated.
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Modules linked in: ust(O-) [last unloaded: fastpath_dummy(O)]
CPU: 0 PID: 159 Comm: modprobe Tainted: G        W  O     N 6.6.49+ #11
Stack:
818bfb70 6093f8d3 0000020d 61381660
61381630 60c1ee6a 00000001 60068c0f
818bfbb0 60983ee6 60983e5c 61381680
Call Trace:
[<60973831>] ? _printk+0x0/0xc2
[<6004b2e6>] show_stack+0x35c/0x382
[<6093f8d3>] ? dump_stack_print_info+0x1af/0x1ec
[<60068c0f>] ? um_set_signals+0x0/0x43
[<60983ee6>] dump_stack_lvl+0x8a/0xa9
[<60983e5c>] ? dump_stack_lvl+0x0/0xa9
[<60068c0f>] ? um_set_signals+0x0/0x43
[<60983f32>] dump_stack+0x2d/0x35
[<60983f05>] ? dump_stack+0x0/0x35
[<6007aad8>] __warn+0x20c/0x294
[<60068c0f>] ? um_set_signals+0x0/0x43
[<60971d98>] warn_slowpath_fmt+0x164/0x189
[<60222128>] ? __update_cpu_freelist_fast+0x96/0xbc
[<60971c34>] ? warn_slowpath_fmt+0x0/0x189
[<6022d2fe>] ? __kmem_cache_free+0x16a/0x1be
[<60068c4a>] ? um_set_signals+0x3b/0x43
[<60941eb4>] ida_free+0x3e0/0x41f
[<605ac993>] misc_minor_free+0x3e/0xbc
[<605acb82>] misc_deregister+0x171/0x1b3
[<81aa7af2>] ustdev_exit+0xa8/0xc1 [ust]

basic code of calling misc_register()/misc_register() is following:

static struct miscdevice ust_dev = {
        0,
        "ustdev",
        &ustdev_ops,
};

int ustdev_init(void)
{
        misc_register(&ust_dev);
        return 0;
}

void ustdev_exit(void)
{
        misc_deregister(&ust_dev);
}

Note that this was working fine without any warning earlier on kernel 6.1.

I suspect it is due to
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=v6.6.51&id=ab760791c0cfbb1d7a668f46a135264f56c8f018.
It seems misc_register() is not calling any ida_allocxxx() api for
static minor value of 0 but misc_deregister() for the same is calling
ida_free() and hence ida_free() is warning in our case.

Please let me know if I am missing something or some of our
assumptions are not valid anymore in newer kernel versions.

Thanks,

Vimal Agrawal