linux-kernel - Re: futex funkiness -- massive lockups

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Date:	Tue, 4 Mar 2014 19:36:55 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: futex funkiness -- massive lockups

On Tue, Mar 4, 2014 at 5:43 PM, Davidlohr Bueso <davidlohr@...com> wrote:
>
>
> From the paths related to futex wait we are stuck when taking the hb
> spinlock in futex_wait_setup >> queue_lock.

Just judging from your trace, I would have suspected a (possibly soft)
lockup in load_balance() rather than the futexes.

The futex being stuck seems expected, since one cpu is definitely
holding the lock - it was interrupted by a timer interrupt at the
successful return case of raw_spin_lock if I read the offset right.

So if that softirq is stuck - perhaps because it's in some endless
loop in load_balance(), or perhaps because it's spending so much time
load-balancing that the next balancing time happens immediately, or
whatever - then you'd see that trace.

              Linus
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