linux-kernel - Re: smp_call_function

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Date:	Mon, 23 Feb 2015 21:50:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Rafael David Tinoco <inaddy@...ntu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jens Axboe <axboe@...nel.dk>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Gema Gomez <gema.gomez-solano@...onical.com>,
	Christopher Arges <chris.j.arges@...onical.com>
Subject: Re: smp_call_function_single lockups

On Mon, Feb 23, 2015 at 11:32:50AM -0800, Linus Torvalds wrote:
> On Mon, Feb 23, 2015 at 6:01 AM, Rafael David Tinoco <inaddy@...ntu.com> wrote:
> >
> > This is v3.19 + your patch (smp acquire/release)
> > - (nested kvm with 2 vcpus on top of proliant with x2apic cluster mode
> > and acpi_idle)
> 
> Hmm. There is absolutely nothing else going on on that machine, except
> for the single call to smp_call_function_single() that is waiting for
> the CSD to be released.
> 
> > * It looks like we got locked because of reentrant flush_tlb_* through
> > smp_call_*
> > but I'll leave it to you.
> 
> No, that is all a perfectly regular callchain:
> 
>  .. native_flush_tlb_others -> smp_call_function_many ->
> smp_call_function_single
> 
> but the stack contains some stale addresses (one is probably just from
> smp_call_function_single() calling into "generic_exec_single()", and
> thus the stack contains the return address inside
> smp_call_function_single() in _addition_ to the actual place where the
> watchdog timer then interrupted it).
> 
> It all really looks very regular and sane, and looks like
> smp_call_function_single() is happily just waiting for the IPI to
> finish in the (inlined) csd_lock_wait().
> 
> I see nothing wrong at all.

[11396.096002] Hardware name: OpenStack Foundation OpenStack Nova, BIOS Bochs 01/01/2011

But its a virtual machine right? Its not running bare metal, its running
a !virt kernel on a virt machine, so maybe some of the virt muck is
borked?

A very subtly broken APIC emulation would be heaps of 'fun'.
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