linux-kernel - Re: [RFC PATCH] char: misc: Init misc->list in a safe way

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Date:   Mon, 26 Jun 2017 14:28:14 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Zhongping Tan (谭中平) 
        <Zhongping.Tan@...eadtrum.com>
Cc:     Orson Zhai (翟京) 
        <Orson.Zhai@...eadtrum.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] char: misc: Init misc->list in a safe way

On Mon, Jun 26, 2017 at 1:48 PM, Zhongping Tan (谭中平)
<Zhongping.Tan@...eadtrum.com> wrote:
> Hi Arnd：
> If we can get list_add(&misc->list, &misc_list), then there is no problem at all, but if the misc_register return "-EBUSY"(Maybe the same miscdevice register twice ), then the deadloop will happen at list_for_each_entry(c, &misc_list, list);
> So at my opinion just remove the initialization code or do initialization when we need do list_add.

I think you are misinterpreting a bug you see: the pointer we pass
into misc_register() must not already be registered, which means that
nothing references misc->list at all.

If misc_register() returns success, and you then call it another time,
you will see the exact behavior that you describe, entering an endless
loop in "list_for_each_entry(c, &misc_list, list)". The correct fix for that
is in the calling code, to ensure that the same device can not get
registered multiple times.

        Arnd