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

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Message-ID: <20160104160924.GA12687@u-isr-ctg-01>
Date:	Mon, 4 Jan 2016 17:09:24 +0100
From:	Dominique Martinet <dominique.martinet@....fr>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Andy Lutomirski <luto@...capital.net>,
	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

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)

> Without DEBUG_MUTEXES there's the MUTEX_SPIN_ON_OWNER code that could
> still confuse things, but that's mutex internal and not scheduler
> related.
> 
> If it ends up being the SPIN_ON_OWNER bits we'll have to cook up some
> extra debug patches.

-- 
Dominique

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