Re: [2.6.33-rc5] tty: possible irq lock inversion dependency intty_fasync

[AmÃrico Wang]

On Sat, Feb 06, 2010 at 10:59:56PM -0800, Linus Torvalds wrote:
>
>
>On Sun, 7 Feb 2010, AmÃrico Wang wrote:
>> 
>> We already fixed this, a better fix:
>
>No we didn't.
>
>> http://lkml.org/lkml/2010/1/26/338
>> 
>> I sent a same fix with Greg's.
>
>We already did that. You didn't read Tetsuo's email carefully.
>
>Let's quote the important parts:
>
>  "is not yet fixed as of 2.6.33-rc7."
>
>and also the _second_ lockdep complaint he quotes, which starts out with
>
>  [   81.651199] =========================================================
>  [   81.651199] [ INFO: possible irq lock inversion dependency detected ]
>  [   81.651199] 2.6.33-rc7 #11
>  [   81.651199] ---------------------------------------------------------
>
>(note the -rc7 there).
>


Ah, I was just mislead by the subject... sigh


>The problem? Look at f_getown: it does
>
>	read_lock(&filp->f_owner.lock);
>
>ie it holds f_owner without interrupts disabled. Now an interrupt comes 
>in, and takes 'siglock' because it ends up sending a signal (timer, SIGIO, 
>whatever). So you have a f_owner -> siglock ordering.
>
>But we _also_ have a siglock -> ctrl_lock -> f_owner ordering, in that 
>problematic tty_fasync() thing. So we have a ABBA deadlock situation.
>
>Yes, it's hard (practically impossible) to trigger, because you have to 
>get an interrupt just at the right point with all the right processes, but 
>lockdep seems to be entirely correct. 
>
>So it is simply _wrong_ to take f_owner while we hold ctrl_lock.
>
>Which is why I suggest just reverting both the original problematic commit 
>_and_ the commit you point to, and just fix the race with that pid_get/put 
>pair instead. As per my patch.
>

Good analysis, this seems better for me too.

Thank you!
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