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

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Date:	Mon, 4 Jan 2016 16:59:15 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Dominique Martinet <dominique.martinet@....fr>,
	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 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).

If its really easy you could start by tracing events/sched/sched_switch
events/sched/sched_wakeup, those would be the actual scheduling events.

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.
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