linux-kernel - Re: [V9fs-developer] Hang triggered by udev coldplug, looks like a race

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Message-ID: <CALCETrVAXFJm7goq-aBo9G=r63WH=FjSRUkzBasGw+VrajzGdg@mail.gmail.com>
Date:	Mon, 4 Jan 2016 11:07:43 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Dominique Martinet <dominique.martinet@....fr>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	V9FS Developers <v9fs-developer@...ts.sourceforge.net>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [V9fs-developer] Hang triggered by udev coldplug, looks like a race

On Mon, Jan 4, 2016 at 8:09 AM, Dominique Martinet
<dominique.martinet@....fr> wrote:
> Peter Zijlstra wrote on Mon, Jan 04, 2016 at 04:59:15PM +0100:
>> On Tue, Dec 29, 2015 at 10:43:26PM -0800, Andy Lutomirski wrote:
>> > [add cc's]
>> >
>> > Hi scheduler people:
>> >
>> > This is relatively easy for me to reproduce.  Any hints for debugging
>> > it?  Could we really have a bug in which processes that are
>> > schedulable as a result of mutex unlock aren't always reliably
>> > scheduled?
>>
>> I would expect that to cause wide-spread fail, then again, virt is known
>> to tickle timing issues that are improbable on actual hardware so
>> anything is possible.
>>
>> Does it reproduce with DEBUG_MUTEXES set? (I'm not seeing a .config
>> here).
>
> The config has CONFIG_DEBUG_MUTEXES=y
>
> It got attached a while ago, reposting it here.
>
>> If its really easy you could start by tracing events/sched/sched_switch
>> events/sched/sched_wakeup, those would be the actual scheduling events.
>
> I'm sure I've missed something in /Documentation but I'm not aware how
> to trace these? (I'm happy to save Andy some precious time as I've got a
> reproducer all set up now)

My reproducer, at least, would make this tricky -- the system ends up
mostly hung, so I don't know how I'd read out the result.  Maybe I'd
try to get something to dump the ftrace buffer to serial console after
a delay and stick all that in initramfs where it wouldn't get stuck
behind the same mutex as everything else.

Or is there a way to tell the kernel to do that for us?

--Andy
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