netdev - Re: [PATCH] netconsole: queue console messages to send later

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Date:	Mon, 07 Jun 2010 15:21:31 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Stephen Hemminger <shemminger@...ux-foundation.org>
Cc:	Flavio Leitner <fleitner@...hat.com>, netdev@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Cong Wang <amwang@...hat.com>, Jay Vosburgh <fubar@...ibm.com>,
	Flavio Leitner <fbl@...close.org>,
	Andy Gospodarek <gospo@...hat.com>,
	Neil Horman <nhorman@...driver.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	bridge@...ts.linux-foundation.org,
	bonding-devel@...ts.sourceforge.net
Subject: Re: [PATCH] netconsole: queue console messages to send later

On Mon, 2010-06-07 at 13:00 -0700, Stephen Hemminger wrote:
> On Mon, 07 Jun 2010 14:50:48 -0500
> Matt Mackall <mpm@...enic.com> wrote:
> 
> > On Mon, 2010-06-07 at 16:24 -0300, Flavio Leitner wrote:
> > > There are some networking drivers that hold a lock in the
> > > transmit path. Therefore, if a console message is printed
> > > after that, netconsole will push it through the transmit path,
> > > resulting in a deadlock.
> > 
> > This is an ongoing pain we've known about since before introducing the
> > netpoll code to the tree.
> > 
> > My take has always been that any form of queueing is contrary to the
> > goal of netpoll: timely delivery of messages even during machine-killing
> > situations like oopses. There may never be a second chance to deliver
> > the message as the machine may be locked solid. And there may be no
> > other way to get the message out of the box in such situations. Adding
> > queueing is a throwing-the-baby-out-with-the-bathwater fix.
> > 
> > I think Dave agrees with me here, and I believe he's said in the past
> > that drivers trying to print messages in such contexts should be
> > considered buggy.
> > 
> 
> Because it to hard to fix all possible device configurations.
> There should be any way to detect recursion and just drop the message to
> avoid deadlock.

Open to suggestions. The locks in question are driver-internal. There
also may not be any actual recursion taking place:

driver path a takes private lock x
driver path a attempts printk
printk calls into netconsole
netconsole calls into driver path b
driver path b attempts to take lock x -> deadlock

So we can't even try to walk back the stack looking for such nonsense.
Though we could perhaps force queuing of all messages -from- the driver
bound to netconsole. Tricky, and not quite foolproof.

-- 
Mathematics is the supreme nostalgia of our time.


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