linux-kernel - Re: [PATCH] Fix Bug messages

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Message-ID: <20080731163515.4a992060@bull.net>
Date:	Thu, 31 Jul 2008 16:35:15 +0200
From:	Sebastien Dugue <sebastien.dugue@...l.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	John Kacur <jkacur@...il.com>,
	Chirag Jog <chirag@...ux.vnet.ibm.com>,
	J?rgen Mell <j.mell@...nline.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Clark Williams <williams@...hat.com>,
	Josh Triplett <josht@...ux.vnet.ibm.com>,
	"Timothy R. Chavez" <tim.chavez@...ux.vnet.ibm.com>
Subject: Re: [PATCH] Fix Bug messages


  Hi Peter,

On Thu, 31 Jul 2008 16:01:47 +0200 Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, 2008-07-31 at 15:49 +0200, John Kacur wrote:
> > Signed-off-by: John Kacur <jkacur@...il.com>
> > Index: linux-2.6.26-rt1/net/core/sock.c
> > ===================================================================
> > --- linux-2.6.26-rt1.orig/net/core/sock.c
> > +++ linux-2.6.26-rt1/net/core/sock.c
> > @@ -1986,11 +1986,12 @@ static __init int net_inuse_init(void)
> >  
> >  core_initcall(net_inuse_init);
> >  #else
> > -static DEFINE_PER_CPU(struct prot_inuse, prot_inuse);
> > +static DEFINE_PER_CPU_LOCKED(struct prot_inuse, prot_inuse);
> >  
> >  void sock_prot_inuse_add(struct net *net, struct proto *prot, int val)
> >  {
> > -       __get_cpu_var(prot_inuse).val[prot->inuse_idx] += val;
> > +       int cpu = 0;
> > +       __get_cpu_var_locked(prot_inuse, cpu).val[prot->inuse_idx] += val;
> >  }
> >  EXPORT_SYMBOL_GPL(sock_prot_inuse_add);
> >  
> > @@ -2000,7 +2001,7 @@ int sock_prot_inuse_get(struct net *net,
> >         int res = 0;
> >  
> >         for_each_possible_cpu(cpu)
> > -               res += per_cpu(prot_inuse, cpu).val[idx];
> > +               res += per_cpu_var_locked(prot_inuse, cpu).val[idx];
> >  
> >         return res >= 0 ? res : 0;
> >  }
> 
> This doesn't look good. You declare it as a PER_CPU_LOCKED, but then
> never use the extra lock to synchronize data.
> 
> Given that sock_proc_inuse_get() is a racy read anyway, the 'right' fix
> would be to do something like:
> 
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 91f8bbc..5a8ace4 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1941,8 +1941,9 @@ static DECLARE_BITMAP(proto_inuse_idx, PROTO_INUSE_NR);
>  #ifdef CONFIG_NET_NS
>  void sock_prot_inuse_add(struct net *net, struct proto *prot, int val)
>  {
> -	int cpu = smp_processor_id();
> +	int cpu = get_cpu();
>  	per_cpu_ptr(net->core.inuse, cpu)->val[prot->inuse_idx] += val;
> +	put_cpu();
>  }
>  EXPORT_SYMBOL_GPL(sock_prot_inuse_add);
>  
> @@ -1988,7 +1989,9 @@ static DEFINE_PER_CPU(struct prot_inuse, prot_inuse);
>  
>  void sock_prot_inuse_add(struct net *net, struct proto *prot, int val)
>  {
> -	__get_cpu_var(prot_inuse).val[prot->inuse_idx] += val;
> +	int cpu = get_cpu();
> +	per_cpu(prot_inuse, cpu).val[prot->inuse_idx] += val;
> +	put_cpu();
>  }
>  EXPORT_SYMBOL_GPL(sock_prot_inuse_add);
>  
> This disables preemption, but only for a very short time - so it doesn't
> hurt the preempt-latency.
> 
> The alternative is to take a lock, do the inc, and drop the lock again,
> which is much more expensive.

  This does indeed seem to fix the problem, no more BUGs appearing
on the console with 2.6.26-rt1 now.

  Thanks,

  Sebastien.


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