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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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