linux-kernel - Re: [PATCH] tty/tty_io.c: make a check before reuse cdev

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Date:	Fri, 25 Jul 2014 23:24:46 +0800
From:	xinhui <mnipxh@...il.com>
To:	Peter Hurley <peter@...leysoftware.com>,
	"xinhui.pan" <xinhuix.pan@...el.com>
CC:	Greg KH <gregkh@...uxfoundation.org>, jslaby@...e.cz,
	linux-kernel@...r.kernel.org, yanmin_zhang@...ux.intel.com
Subject: Re: [PATCH] tty/tty_io.c: make a check before reuse cdev

hi, Peter

On 2014年07月24日 00:04, Peter Hurley wrote:
> Hi Xinhui,
> 
> On 07/23/2014 05:21 AM, xinhui.pan wrote:
>> 于 2014年07月23日 00:40, Peter Hurley 写道:
>>> On 07/22/2014 07:52 AM, xinhui.pan wrote:
>>>> 于 2014年07月21日 23:38, Greg KH 写道:
>>>>> On Mon, Jul 21, 2014 at 08:47:16PM +0800, pp wrote:
> 
>>>>>> tty driver register its device and (D)init the cdevs again.
>>>>>
>>>>> What driver does this with an "old" device, it should have created a new
>>>>> one, otherwise, as you have pointed out, it's a bug.
>>>>>
>>>>
>>>> I can't agree more with you. we should not use "old" device.
>>>
>>> This is a gsm driver problem. The GSM driver is reusing device indexes
>>> for still-open ttys.
>>>
>>> The GSM driver uses a global table, gsm_mux[], to allocate device indexes
>>> but prematurely clears the table entry in gsm_mux_cleanup(). If instead,
>>> clearing the gsm_mux table entry were deferred to gsm_mux_free(), then
>>> device indexes would not be getting reused until after the last tty
>>> associated with the last gsm attach was closed.
>>>
>>
>> Very nice solution. We will check if this can cause any risk, both to kernel and user space.
>> Using a new tty base to register with new cdevs may give us more chance to wait PROCESS quit/close.
>> when total 256 tty used up, what we should do is still in discuss.
> 
> I saw your patch for the use of gsm->num before gsm_activate_mux() has
> allocated the table entry; thanks for fixing that.
> 
> As for what to do if all the gsm_mux table entries are used: if the error
> is infrequent, I suggest simply returning an error which is what the
> driver does currently. Otherwise, a more dynamic allocation scheme may be required.
> 
> I did notice while reviewing the error handling that gsmld_open() will
> leak the entire composite ldisc data allocated by gsm_alloc_mux() if
> gsmld_attach_gsm() fails.
> 

As for memory leak, I suggest move the codes that changing the index of gsm_mux[] into gsm_alloc_mux/gsm_free_mux.
That makes things much easier to understand. And everything should be OK, including base = gsm->num << 6 :)

I think gsm_mux[] holds the gsms whcih we are able to use, not we are using. *able to use* includes *using*.

If you have any different opinions, could you point out my mistake?

thanks,

xinhui

> Regards,
> Peter Hurley
> 
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