linux-kernel - Re: sched: long running interrupts breaking spinlocks

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Message-ID: <20140408161023.GP10526@twins.programming.kicks-ass.net>
Date:	Tue, 8 Apr 2014 18:10:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: sched: long running interrupts breaking spinlocks

On Tue, Apr 08, 2014 at 11:26:56AM -0400, Sasha Levin wrote:
> Hi all,
> 
> (all the below happened inside mm/ code, so while I don't suspect
> it's a mm/ issue you folks got cc'ed anyways!)
> 
> While fuzzing with trinity inside a KVM tools guest running the latest -next
> kernel, I've stumbled on the following:
> 
> [ 4071.166362] BUG: spinlock lockup suspected on CPU#19, trinity-c19/17092

That's a heuristic in the spinlock code; triggering it with big machines
(19 cpus is far bigger than anything at the time that code was written)
and virt (yay for lock owner preemption; another thing we didn't have
back when) is trivial.

I'd not worry too much about this.

So DEBUG_SPINLOCKS turns spin_lock() into something like:

  for (i = 0; i < loops; i++)
  	if (spin_trylock())
		return;

  /* complain */

And you simply ran out of loops.
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