linux-kernel - Re: [PATCH] char: misc: make misc_open() and misc

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Date:   Mon, 4 Jul 2022 23:07:54 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Wedson Almeida Filho <wedsonaf@...gle.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        arnd@...db.de, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH] char: misc: make misc_open() and misc_register() killable

On 2022/07/04 22:57, Wedson Almeida Filho wrote:
> On Mon, Jul 04, 2022 at 10:48:32PM +0900, Tetsuo Handa wrote:
>> On 2022/07/04 21:59, Wedson Almeida Filho wrote:
>>>> @@ -139,6 +139,10 @@ static int misc_open(struct inode *inode, struct file *file)
>>>>  
>>>>  	err = 0;
>>>>  	replace_fops(file, new_fops);
>>>> +	if (iter->unlocked_open && file->f_op->open) {
>>>> +		mutex_unlock(&misc_mtx);
>>>> +		return file->f_op->open(inode, file);
>>>> +	}
>>>
>>> One of the invariants of miscdev is that once misc_deregister() returns,
>>> no new calls to f_op->open() are made. (Although, of course, you can
>>> still have open files but that's a whole different problem.)
>>
>> The point of this change is that file->f_op after mutex_unlock(&misc_mtx) is
>>  from new_fops which is guaranteed to hold a ref on "struct file_operations"
>> via new_fops = fops_get("struct miscdevice"->fops).
>> That is, a module ref remains valid even after mutex_unlock(&misc_mtx).
>>
>> And as with major_names_lock case quoted below, this change assumes that
>> misc_deregister() is called from module's __exit function, and fops_get()
>> is preventing the module owning new_fops from calling __exit function.
> 
> Your assumption is not sound. misc_deregister() can be (and is)
> legitimately called from other places, for example, a driver's remove()
> callback. In fact, when I grep for misc_deregister(), the second
> instance is such a case.

OK, the frequency of calling misc_deregister() can be much higher than
unregister_blkdev(), which means that misc_mtx is more prone to trigger
hung task warnings. I'm more inclined to avoid sleeping with misc_mtx held.