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

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Date:	Thu, 31 Jul 2008 16:10:21 +0200
From:	"John Kacur" <jkacur@...il.com>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	"Sebastien Dugue" <sebastien.dugue@...l.net>,
	"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

On Thu, Jul 31, 2008 at 4:01 PM, 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.
>
>

Cool, thanks for the quick feedback. What kind of criteria are used to
decide between disabling preemption for a short time, or using the
more expensive lock?
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