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

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Date:	Thu, 31 Jul 2008 16:01:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	John Kacur <jkacur@...il.com>
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, 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.

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