netdev - Re: [PATCH] lockdep: fix sk->sk_callback

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Message-Id: <1164800544.6588.118.camel@twins>
Date:	Wed, 29 Nov 2006 12:42:24 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, akpm@...l.org, mingo@...e.hu,
	gandalf@...g.westbo.se
Subject: Re: [PATCH] lockdep: fix sk->sk_callback_lock locking

On Wed, 2006-11-29 at 18:49 +1100, Herbert Xu wrote:
> Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > 
> > =========================================================
> > [ INFO: possible irq lock inversion dependency detected ]
> > 2.6.19-rc6 #4
> > ---------------------------------------------------------
> > nc/1854 just changed the state of lock:
> > (af_callback_keys + sk->sk_family#2){-.-?}, at: [<c0268a7f>] sock_def_error_report+0x1f/0x90
> > but this lock was taken by another, soft-irq-safe lock in the past:
> > (slock-AF_INET){-+..}
> > 
> > and interrupts could create inverse lock ordering between them.
> 
> I think this is bogus.  The slock is not a standard lock.  When we
> hold it in process context we don't actually hold the spin lock part
> of it.  However, it does prevent the softirq path from running in
> critical sections which also prevents any attempt to grab the
> callback lock from softirq context.
> 
> If you still think there is a problem, please show an actual scenario
> where it dead locks.

process context does lock_sock(sk) which is basically a sleeping lock
and sets an owner field when acquired.

BH context does bh_lock_sock(sk); which spins on the spinlock protecting
the owner field; and checks for an owner under this lock. When an owner
is found it will stick the skb on a queue for later processing.

This scheme does indeed seem to avoid the reported deadlock scenario -
although I didn't audit all code paths.

However I'm not quite sure yet how to teach lockdep about this. The
proposed patch will shut it up though.

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