linux-kernel - Re: cleanup_net()/net_mutex hung tasks + kobject release debugging

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Date:	Thu, 21 Jul 2016 09:13:46 -0400
From:	Dave Jones <davej@...emonkey.org.uk>
To:	Vegard Nossum <vegard.nossum@...cle.com>
Cc:	trinity@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Russell King <rmk+kernel@....linux.org.uk>
Subject: Re: cleanup_net()/net_mutex hung tasks + kobject release debugging

On Thu, Jul 21, 2016 at 02:43:34PM +0200, Vegard Nossum wrote:

 > The rules for net_mutex are very simple, it's used in very few places so
 > I don't see how the locking could get messed up there. I'll buy your
 > theory that the lock is held for a long time if there are a lot of
 > namespaces to iterate over. I decided to time it myself and it seems
 > that cleanup_net() can hold the mutex for 30-40 seconds at a time, which
 > is surely wrong.

 > so on a hunch I disabled DEBUG_KOBJECT_RELEASE, and that does indeed
 > solve the problem -- cleanup_net() still holds the mutex for fairly
 > long, but only up to max ~5 seconds at a time as opposed to 30-40.

Yeah, I never ran with that option enabled (it used to cause my testbox
to not boot, and I never got around to debugging why). I thought five seconds
was painful enough. I guess we have different thresholds for acceptable
behaviour here :-)

Could be one of the other debug options I had enabled exacerbates the
cleanup_net problem in a similar way though.

 > There's maybe a case for cleanup_net() to release the mutex every now
 > and again during cleanup, but I was also seeing a few other hung tasks
 > unrelated to net_mutex when I disabled the unshare() system call in
 > trinity, which makes me wonder if we need a more general solution.

Not sure. We may have to just look at these on a case by case basis.

	Dave