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SubjectRe: [PATCH] netfilter: iptables: lock free counters, PREEMPT_RCU=y fix

* Stephen Hemminger <shemminger@vyatta.com> wrote:

> On Thu, 2 Apr 2009 22:12:45 +0200
> Ingo Molnar <mingo@elte.hu> wrote:
>
> >
> > Impact: fix log spam under CONFIG_DEBUG_PREEMPT=y
> >
> > This recent commit:
> >
> > 7845447: netfilter: iptables: lock free counters
> >
> > Converted a couple of netfilter codepaths from read_lock() critical
> > sections to lockless rcu_read_lock(). What it forgot about is that
> > under CONFIG_PREEMPT=y and CONFIG_PREEMPT_RCU=y these sections can
> > be preempted.
> >
> > Under CONFIG_DEBUG_PREEMPT=y this produces such warnings:
> >
> > BUG: using smp_processor_id() in preemptible [00000000] code: ssh/9115
> > caller is ipt_do_table+0xc8/0x559
> > Pid: 9115, comm: ssh Tainted: G W 2.6.29-tip-08646-g45ef7c3-dirty #26231
> > Call Trace:
> > [<c0c0dacf>] ? printk+0x14/0x16
> > [<c048c2e6>] debug_smp_processor_id+0xa6/0xbc
> > [<c0adbd03>] ipt_do_table+0xc8/0x559
> > [<c0c10277>] ? _read_unlock+0x3d/0x49
> > [<c0ad68c0>] ? fn_hash_lookup+0x94/0xa0
> > [<c0ad330e>] ? __inet_dev_addr_type+0x56/0x8d
> > [<c0a87b02>] ? neigh_lookup+0xe5/0x108
> > [<c0adc2bc>] ipt_local_hook+0x40/0x50
> > [<c0a93e57>] nf_iterate+0x34/0x80
> > [<c0aad4b8>] ? dst_output+0x0/0x10
> > [<c0a93eea>] nf_hook_slow+0x47/0xa4
> > [<c0aad4b8>] ? dst_output+0x0/0x10
> > [<c0aaec04>] __ip_local_out+0x78/0x7f
> > [<c0aad4b8>] ? dst_output+0x0/0x10
> > [<c0aaec1b>] ip_local_out+0x10/0x20
> > [<c0aaf416>] ip_queue_xmit+0x2bc/0x332
> > [<c0aa8fdf>] ? __ip_route_output_key+0x112/0x77b
> > [<c01397fe>] ? local_bh_enable+0x10/0x12
> > [<c0abf0e5>] ? tcp_connect+0x32a/0x3bb
> > [<c0ab15a7>] ? __inet_hash_nolisten+0x97/0xaf
> > [<c0a78dde>] ? __copy_skb_header+0xe/0x13a
> > [<c0abf0e5>] ? tcp_connect+0x32a/0x3bb
> > [<c0abe955>] ? tcp_transmit_skb+0x5a5/0x61c
> > [<c0abe995>] tcp_transmit_skb+0x5e5/0x61c
> > [<c0a7b6c0>] ? __alloc_skb+0x54/0x120
> > [<c0abefca>] ? tcp_connect+0x20f/0x3bb
> > [<c0abf0e5>] tcp_connect+0x32a/0x3bb
> > [<c0ac4b68>] tcp_v4_connect+0x466/0x4be
> > [<c0acf380>] inet_stream_connect+0x8f/0x212
> > [<c018f4e6>] ? might_fault+0x75/0x77
> > [<c047f198>] ? copy_from_user+0x2f/0x117
> > BUG: using smp_processor_id() in preemptible [00000000] code: ssh/9114
> >
> > Since it appears that the tables are RCU freed, and there are no
> > non-preempt assumptions in the code, the using of
> > raw_smp_processor_id() is safe.
> >
> > [ I also audited all of net/netfilter/*.c for smp_processor_id() use,
> > and fixed all places that used them unsafely. ]
> >
> > Signed-off-by: Ingo Molnar <mingo@elte.hu>
> > ---
> > net/ipv4/netfilter/arp_tables.c | 2 +-
> > net/ipv4/netfilter/ip_tables.c | 4 ++--
> > 2 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/net/ipv4/netfilter/arp_tables.c b/net/ipv4/netfilter/arp_tables.c
> > index 35c5f6a..30baf3e 100644
> > --- a/net/ipv4/netfilter/arp_tables.c
> > +++ b/net/ipv4/netfilter/arp_tables.c
> > @@ -255,7 +255,7 @@ unsigned int arpt_do_table(struct sk_buff *skb,
> >
> > rcu_read_lock();
> > private = rcu_dereference(table->private);
> > - table_base = rcu_dereference(private->entries[smp_processor_id()]);
> > + table_base = rcu_dereference(private->entries[raw_smp_processor_id()]);
> >
> > e = get_entry(table_base, private->hook_entry[hook]);
> > back = get_entry(table_base, private->underflow[hook]);
> > diff --git a/net/ipv4/netfilter/ip_tables.c b/net/ipv4/netfilter/ip_tables.c
> > index 82ee7c9..eff124e 100644
> > --- a/net/ipv4/netfilter/ip_tables.c
> > +++ b/net/ipv4/netfilter/ip_tables.c
> > @@ -280,7 +280,7 @@ static void trace_packet(struct sk_buff *skb,
> > char *hookname, *chainname, *comment;
> > unsigned int rulenum = 0;
> >
> > - table_base = (void *)private->entries[smp_processor_id()];
> > + table_base = (void *)private->entries[raw_smp_processor_id()];
> > root = get_entry(table_base, private->hook_entry[hook]);
> >
> > hookname = chainname = (char *)hooknames[hook];
> > @@ -341,7 +341,7 @@ ipt_do_table(struct sk_buff *skb,
> >
> > rcu_read_lock();
> > private = rcu_dereference(table->private);
> > - table_base = rcu_dereference(private->entries[smp_processor_id()]);
> > + table_base = rcu_dereference(private->entries[raw_smp_processor_id()]);
> >
> > e = get_entry(table_base, private->hook_entry[hook]);
> >
>
> NAK. The rcu_read_lock() needs to be rcu_read_lock_bh() otherwise RCU
> could corrupt the referenceses.

Indeed - somehow i missed that it originally used read_lock_bh().

I was wondering about that already: how could get_entry() and
nf-iterator work without worrying about softnet-rx processing, it
just looked too involved to be trivially preemptible.

Thanks, i'll use Eric's patch.

Ingo


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