linux-kernel - Re: Lockdep problem

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Message-ID: <52CEF521.90009@lwfinger.net>
Date:	Thu, 09 Jan 2014 13:14:41 -0600
From:	Larry Finger <Larry.Finger@...inger.net>
To:	Stanislaw Gruszka <sgruszka@...hat.com>
CC:	linux-wireless <linux-wireless@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Lockdep problem

On 01/09/2014 10:24 AM, Stanislaw Gruszka wrote:
> On Tue, Jan 07, 2014 at 12:28:26PM -0600, Larry Finger wrote:
>>    rtnl_mutex --> misc_mtx --> rfkill_global_mutex
>>
>>    Possible unsafe locking scenario:
>>
>>          CPU0                    CPU1
>>          ----                    ----
>>     lock(rfkill_global_mutex);
>>                                  lock(misc_mtx);
>>                                  lock(rfkill_global_mutex);
>>     lock(rtnl_mutex);
>>
>>   *** DEADLOCK ***
>
> There are 3 mutexes dependency. The deadlock can happen if on another
> cpu, let say CPU2, there will be sequence:
>
> lock(rtnl_mutex);
> lock(misc_mtx);
>
> Then on deadlock scenario:
> CPU0 waits for rtnl_mutex to unlock, keep rfkill_global_mutex locked
> CPU1 waits for rfkill_global_mutex to unlock, keep misc_mtx locked
> CPU2 waits for misc_mtx to unlock, keep rtnl_mutex locked.
>
> This dependency can be broken by moving b43_rng_init() outside from
> rtnl_mutex scope, like on below patch. IIUC b43 random number generator
> works only if we already started network connection, but this should
> not be a problem, as b43_rng_read() do not return any data if device
> is not prepared.
>
> You could also remove whole b43 rnd. Everyone know, that HW vendors
> are influenced by NSA and they random generators do not provide truly
> random numbers (just kidding ;-)

Well, I have no idea to what extent NSA affects the RNGs in computers; however, 
I do like having that additional source of entropy. :)

Your analysis was spot on, and I will be submitting a patch shortly with you as 
author. I'll give the reporter on b.k.o a chance to test it, but it removed the 
splat here.

Thanks,

Larry


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