netdev - Re: Kernel WARNING: at net/core/dev.c:1330 __netif

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Date:	Thu, 24 Jul 2008 21:06:51 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	peterz@...radead.org, davem@...emloft.net, jarkao2@...il.com,
	Larry.Finger@...inger.net, kaber@...sh.net,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-wireless@...r.kernel.org, mingo@...hat.com,
	paulmck@...ux.vnet.ibm.com
Subject: Re: Kernel WARNING: at net/core/dev.c:1330 __netif_schedule+0x2c/0x98()

On Thursday 24 July 2008 20:55, Miklos Szeredi wrote:
> On Thu, 24 Jul 2008, Nick Piggin wrote:
> > Hey, something kind of cool (and OT) I've just thought of that we can
> > do with ticket locks is to take tickets for 2 (or 64K) nested locks,
> > and then wait for them both (all), so the cost is N*lock + longest spin,
> > rather than N*lock + N*avg spin.
>
> Isn't this deadlocky?
>
> E.g. one task takes ticket x=1, then other task comes in and takes x=2
> and y=1, then first task takes y=2.  Then neither can actually
> complete both locks.

Oh duh of course you still need mutual exclusion from the first lock
to order the subsequent :P

So yeah it only works for N > 2 locks, and you have to spin_lock the
first one... so unsuitable for scheduler.
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