linux-kernel - Re: [PATCH v3 2/3] hvc_init(): Enforce one-time initialization.

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Date:	Mon, 5 Dec 2011 16:24:52 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Miche Baker-Harvey <miche@...gle.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	xen-devel@...ts.xensource.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Anton Blanchard <anton@...ba.org>,
	Mike Waychison <mikew@...gle.com>,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	Eric Northrup <digitaleric@...gle.com>
Subject: Re: [PATCH v3 2/3] hvc_init(): Enforce one-time initialization.

On (Tue) 29 Nov 2011 [09:50:41], Miche Baker-Harvey wrote:
> Good grief!  Sorry for the spacing mess-up!  Here's a resend with reformatting.
> 
> Amit,
> We aren't using either QEMU or kvmtool, but we are using KVM.  All

So it's a different userspace?  Any chance this different userspace is
causing these problems to appear?  Esp. since I couldn't reproduce
with qemu.

> the issues we are seeing happen when we try to establish multiple
> virtioconsoles at boot time.  The command line isn't relevant, but I
> can tell you the protocol that's passing between the host (kvm) and
> the guest (see the end of this message).
> 
> We do go through the control_work_handler(), but it's not
> providing synchronization.  Here's a trace of the
> control_work_handler() and handle_control_message() calls; note that
> there are two concurrent calls to control_work_handler().

Ah; how does that happen?  control_work_handler() should just be
invoked once, and if there are any more pending work items to be
consumed, they should be done within the loop inside
control_work_handler().

> I decorated control_work_handler() with a "lifetime" marker, and
> passed this value to handle_control_message(), so we can see which
> control messages are being handled from which instance of
> the control_work_handler() thread.
> 
> Notice that we enter control_work_handler() a second time before
> the handling of the second PORT_ADD message is complete. The
> first CONSOLE_PORT message is handled by the second
> control_work_handler() call, but the second is handled by the first
> control_work_handler() call.
> 
> root@...buntu:~# dmesg | grep MBH
> [3371055.808738] control_work_handler #1
> [3371055.809372] + #1 handle_control_message PORT_ADD
> [3371055.810169] - handle_control_message PORT_ADD
> [3371055.810170] + #1 handle_control_message PORT_ADD
> [3371055.810244]  control_work_handler #2
> [3371055.810245] + #2 handle_control_message CONSOLE_PORT
> [3371055.810246]  got hvc_ports_mutex
> [3371055.810578] - handle_control_message PORT_ADD
> [3371055.810579] + #1 handle_control_message CONSOLE_PORT
> [3371055.810580]  trylock of hvc_ports_mutex failed
> [3371055.811352]  got hvc_ports_mutex
> [3371055.811370] - handle_control_message CONSOLE_PORT
> [3371055.816609] - handle_control_message CONSOLE_PORT
> 
> So, I'm guessing the bug is that there shouldn't be two instances of
> control_work_handler() running simultaneously?

Yep, I assumed we did that but apparently not.  Do you plan to chase
this one down?

		Amit
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