linux-kernel - Re: Linux 3.16-rc6

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Date:	Thu, 24 Jul 2014 10:41:27 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	USB list <linux-usb@...r.kernel.org>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>
Subject: Re: Linux 3.16-rc6

On Thu, Jul 24, 2014 at 08:43:53AM +0200, Peter Zijlstra wrote:
> 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@...en8.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@....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.

CCing original author:

@Waiman, you can easily reproduce by booting a kvm guest with rc6 +
tip/master. It does not trigger everytime so you need to try a couple of
iterations.

I'm attaching my .config.

Also, here the splats I'm seeing on my machines:

https://lkml.kernel.org/r/20140722081840.GA6462@pd.tnic
https://lkml.kernel.org/r/20140723095327.GA23131@pd.tnic

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
--

View attachment ".config" of type "text/plain" (87338 bytes)