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Date: Wed, 23 Jul 2014 18:53:17 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
cc: Borislav Petkov <bp@alien8.de>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>, Ming Lei <tom.leiming@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org,
        linux-input@vger.kernel.org
Subject: Re: Linux 3.16-rc6
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References: <CA+55aFzJKxzw_as9iGgeM=MdQ6trUkQLDuoEr8nxj88sOqQnQA@mail.gmail.com> <20140723095327.GA23131@pd.tnic> <CA+55aFzPs4uF=eR7jCnpyBtgWNRkX-uDfXeo5u34VEvT8_yyzw@mail.gmail.com>
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On Wed, 23 Jul 2014, Linus Torvalds wrote:

> > Well, it looks like we f*cked up something after -rc5 since I'm starting
> > to see lockdep splats all over the place which I didn't see before. I'm
> > running rc6 + tip/master.
> >
> > There was one in r8169 yesterday:
> >
> > https://lkml.kernel.org/r/20140722081840.GA6462@pd.tnic
> >
> > and now I'm seeing the following in a kvm guest. I'm adding some more
> > lists to CC which look like might be related, judging from the stack
> > traces.
> 
> Hmm. I'm not seeing the reason for this.
> 
> > [   31.704282] [ INFO: possible irq lock inversion dependency detected ]
> > [   31.704282] 3.16.0-rc6+ #1 Not tainted
> > [   31.704282] ---------------------------------------------------------
> > [   31.704282] Xorg/3484 just changed the state of lock:
> > [   31.704282]  (tasklist_lock){.?.+..}, at: [<ffffffff81184b19>] send_sigio+0x59/0x1b0
> > [   31.704282] but this lock took another, HARDIRQ-unsafe lock in the past:
> > [   31.704282]  (&(&p->alloc_lock)->rlock){+.+...}
> 
> Ok, so the claim is that there's a 'p->alloc_lock' (ie "task_lock()")
> that is inside the tasklist_lock, which would indeed be wrong. But I'm
> not seeing it. The "shortest dependencies" thing seems to imply
> __set_task_comm(), but that only takes task_lock.
> 

It's the reverse, task_lock() inside tasklist_lock is fine but it's 
complaining about taking tasklist_lock inside task_lock().

I don't think it's anything that's sitting in tip/master nor is it 
something that was introduced during this merge window.  I think this has 
been the behavior dating back to commit 94dfd7edfd5c ("USB: HCD: support 
giveback of URB in tasklet context").
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