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Date: Thu, 24 Jul 2014 08:43:53 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>,
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Subject: Re: Linux 3.16-rc6
Message-ID: <20140724064353.GW9918@twins.programming.kicks-ass.net>
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 <20140723095327.GA23131@pd.tnic>
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On Wed, Jul 23, 2014 at 05:37:43PM -0700, Linus Torvalds wrote:
> On Wed, Jul 23, 2014 at 2:53 AM, Borislav Petkov <bp@alien8.de> 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.
> 
> Unless there is something in tip/master. 

lkml.kernel.org/r/tip-e0645a111cb44e01adc6bfff34f683323863f4d2@git.kernel.org

Its supposed to change lockdep to the stricter semantics provided by the
qrwlock.

Where the rwlock used to be unfair and reader biased, qrwlock is 'fair'
and only allows interrupt recursion.

> Can you check that this is
> actually in plain -rc6?
> 
> Or maybe I'm just blind. Those lockdep splats are easy to get wrong.
> Adding PeterZ and Ingo to the list just because they are my lockdep
> go-to people.

I've been staring at this splat from borislav since yesterday morning
and confusing myself properly.. I'll continue doing so until I'm
decided.
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