Hi Peter,
On Thu, 31 Jul 2008 16:01:47 +0200 Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, 2008-07-31 at 15:49 +0200, John Kacur wrote:
> > Signed-off-by: John Kacur <jkacur@xxxxxxxxx>
> > 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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