Messages in this thread Patch in this message | | | Subject | Re: [LOCKDEP BUG][2.6.36-rc1] xt_info_wrlock? | From | Eric Dumazet <> | Date | Mon, 16 Aug 2010 20:48:05 +0200 |
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Le lundi 16 août 2010 à 14:16 -0400, Steven Rostedt a écrit :
> We need a better comment than that. Could that be changed to something like: > > /* > * lockdep tests if we grab a lock and can be preempted by > * a softirq and that softirq grabs the same lock causing a > * deadlock. > * This is a special case because this is a per-cpu lock, > * and we are only grabbing the lock for other CPUs. A softirq > * will only takes its local CPU lock thus, if we are preempted > * by a softirq, then it will grab the current CPU lock which > * we do not take here. > * > * Simply disable lockdep here until it can handle this situation. > */ >
You mean duplicating this long comment in three files, or only once in Changelog ?
My choice was to document the lockdep_off() use (very seldom used in kernel) in the Changelog. Hopefully, people messing with this code know about git ;)
I agree I didnt document how netfilter locks work in this Changelog.
And in original commit (24b36f019) I forgot to state that get_counters() is guarded by a mutex, so that no more than one cpu runs in get_counters().
What about following ?
[PATCH] netfilter: {ip,ip6,arp}_tables: avoid lockdep false positive
After commit 24b36f019 (netfilter: {ip,ip6,arp}_tables: dont block bottom half more than necessary), lockdep can raise a warning because we attempt to lock a spinlock with BH enabled, while the same lock is usually locked by another cpu in a softirq context.
In this use case, the lockdep splat is a false positive, because the BH disabling only matters for one cpu and its associated.
1) We use one spinlock per cpu.
2) A softirq will only lock the lock associated to current cpu.
3) get_counters() disables sofirqs while fetching data of current cpu. (to avoid a deadlock if a softirq comes and try to lock same lock)
4) other locks are locked without blocking softirq (as a softirq will lock another lock)
5) get_counter() calls are serialized by a mutex.
This to avoid a deadlock if two cpus were doing :
CPU1 CPU2 lock lock#1 lock lock#2 copy data#1 copy data#2 unlock lock#1 unlock lock#2
lock#2 lock#1 softirq lock#1 softirq, attempt to lock lock#2 <deadlock> <deadlock>
Use lockdep_off()/lockdep_on() around the problematic section to avoid the splat.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org> Diagnosed-by: David S. Miller <davem@davemloft.net> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com> CC: Patrick McHardy <kaber@trash.net> --- net/ipv4/netfilter/arp_tables.c | 3 +++ net/ipv4/netfilter/ip_tables.c | 3 +++ net/ipv6/netfilter/ip6_tables.c | 3 +++ 3 files changed, 9 insertions(+)
diff --git a/net/ipv4/netfilter/arp_tables.c b/net/ipv4/netfilter/arp_tables.c index 6bccba3..b4f7ebf 100644 --- a/net/ipv4/netfilter/arp_tables.c +++ b/net/ipv4/netfilter/arp_tables.c @@ -729,8 +729,10 @@ static void get_counters(const struct xt_table_info *t, local_bh_enable(); /* Processing counters from other cpus, we can let bottom half enabled, * (preemption is disabled) + * We must turn off lockdep to avoid a false positive. */ + lockdep_off(); for_each_possible_cpu(cpu) { if (cpu == curcpu) continue; @@ -743,6 +745,7 @@ static void get_counters(const struct xt_table_info *t, } xt_info_wrunlock(cpu); } + lockdep_on(); put_cpu(); } diff --git a/net/ipv4/netfilter/ip_tables.c b/net/ipv4/netfilter/ip_tables.c index c439721..dc5b2fd 100644 --- a/net/ipv4/netfilter/ip_tables.c +++ b/net/ipv4/netfilter/ip_tables.c @@ -903,8 +903,10 @@ get_counters(const struct xt_table_info *t, local_bh_enable(); /* Processing counters from other cpus, we can let bottom half enabled, * (preemption is disabled) + * We must turn off lockdep to avoid a false positive. */ + lockdep_off(); for_each_possible_cpu(cpu) { if (cpu == curcpu) continue; @@ -917,6 +919,7 @@ get_counters(const struct xt_table_info *t, } xt_info_wrunlock(cpu); } + lockdep_on(); put_cpu(); } diff --git a/net/ipv6/netfilter/ip6_tables.c b/net/ipv6/netfilter/ip6_tables.c index 5359ef4..fb55443 100644 --- a/net/ipv6/netfilter/ip6_tables.c +++ b/net/ipv6/netfilter/ip6_tables.c @@ -916,8 +916,10 @@ get_counters(const struct xt_table_info *t, local_bh_enable(); /* Processing counters from other cpus, we can let bottom half enabled, * (preemption is disabled) + * We must turn off lockdep to avoid a false positive. */ + lockdep_off(); for_each_possible_cpu(cpu) { if (cpu == curcpu) continue; @@ -930,6 +932,7 @@ get_counters(const struct xt_table_info *t, } xt_info_wrunlock(cpu); } + lockdep_on(); put_cpu(); }
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