linux-kernel - Re: [PATCH 4/7] hvc_console: Fix race between hvc_close and hvc

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Message-ID: <4BAC9D9E.5070901@in.ibm.com>
Date:	Fri, 26 Mar 2010 17:12:22 +0530
From:	Sachin Sant <sachinp@...ibm.com>
To:	Amit Shah <amit.shah@...hat.com>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Greg Kroah-Hartman <gregkh@...e.de>, a.p.zijlstra@...llo.nl,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	Anton Blanchard <anton@...ba.org>, tglx@...utronix.de,
	Linus Torvalds <torvalds@...ux-foundation.org>, mingo@...e.hu,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH 4/7] hvc_console: Fix race between hvc_close and hvc_remove

Amit Shah wrote:
> On (Fri) Mar 26 2010 [14:43:56], Sachin Sant wrote:
>   
>>> And this suggests that hvc_kick() is called before hvc_task is
>>> initialised, ie, before hvc_init() is called.
>>>
>>> Does this help?
>>>   
>>>       
>> Hi Amit,
>>
>> I too ran into this issue as reported by Anton. Unfortunately in my
>> case the following suggested patch does not help boot the powerpc
>> box successfully. The boot still hangs with following messages
>>     
>
> Was the hang you saw before applying this patch at the same place / for
> the same reason?
>   
Yes the machine hangs at the same place with or without the patch.

>> mount: can't find /root/proc in /etc/fstab or /etc/mtab
>> /sbin/smart_agetty: can not determine 'console' speed
>>     
>
> OK, this gets even murkier. I don't know why taking a few krefs modifies
> hvc_console behaviour to such an extent.
>   
Looking at the commit e74d098c66543d0731de62eb747ccd5b636a6f4c,
i see that for every tty_kref_get() there is a corresponding
tty_kref_put() except maybe for the one in the following patch snippet

        spin_lock_irqsave(&hp->lock, flags);
        /* Check and then increment for fast path open. */
        if (hp->count++ > 0) {
+               tty_kref_get(tty);
                spin_unlock_irqrestore(&hp->lock, flags);
                hvc_kick();
                return 0;

I don't know this code very well but we might be missing a
corresponding tty_kref_put() some place ?

If i comment out the above tty_kref_get() call, the machine
boots fine with or without the suggested patch.

Thanks
-Sachin


>   
>> Just to confirm i too am facing same issue as reported by Anton,
>> reverted commit e74d098c66543d0731de62eb747ccd5b636a6f4c and that allowed
>> by box to boot successfully.
>>     
>
> Which module uses hvc_console in your case?
>
> 		Amit
>
>   


-- 

---------------------------------
Sachin Sant
IBM Linux Technology Center
India Systems and Technology Labs
Bangalore, India
---------------------------------

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