The per-CPU xt_recseq is a custom netfilter seqcount. It provides synchronisation for the replacement of the xt_table::private pointer and ensures that the two counter in xt_counters are properly observed during an update on 32bit architectures. xt_recseq also supports recursion. This construct is less than optimal on PREMPT_RT because the lack of an associated lock (with the seqcount) can lead to a deadlock if a high priority reader interrupts a writter. Also xt_recseq relies on locking with BH-disable which becomes problematic if the lock, currently part of local_bh_disable() on PREEMPT_RT, gets removed. This can be optimized unrelated to PREEMPT_RT: - Use RCU for synchronisation. This means ipt_do_table() (and the two other) access xt_table::private within a RCU section. xt_replace_table() replaces the pointer with rcu_assign_pointer() and uses synchronize_rcu() to wait until each reader left RCU section. - Use u64_stats_t for the statistics. The advantage here is that u64_stats_sync which is use a seqcount is optimized away on 64bit architectures. The increment becomes just an add, the read just a read of the variable without a loop. On 32bit architectures the seqcount remains but the scope is smaller. The struct xt_counters is defined in a user exported header (uapi). So in patch #2 I tried to split the regular u64 access and the "internal access" which treats the struct either as two counter or a per-CPU pointer. In order not to expose u64_stats_t to userland I added a "pad" which is cast to the internal type. I hoped that this makes it obvious that a function like xt_get_this_cpu_counter() expects the possible per-CPU type but mark_source_chains() or get_counters() expect the u64 type without pointers. v1…v2 https://lore.kernel.org/all/20250216125135.3037967-1-bigeasy@xxxxxxxxxxxxx/ - Updated kerneldoc in 2/3 so that the renamed parameter is part of it. - Updated description 1/3 in case there are complains regarding the synchronize_rcu(). The suggested course of action is to motivate people to move away from "legacy" towards "nft" tooling. Last resort is not to wait for the in-flight counter and just copy what is there. Sebastian Andrzej Siewior (3): netfilter: Make xt_table::private RCU protected. netfilter: Split the xt_counters type between kernel and user. netfilter: Use u64_stats for counters in xt_counters_k. include/linux/netfilter/x_tables.h | 113 +++++++----------- include/uapi/linux/netfilter/x_tables.h | 4 + include/uapi/linux/netfilter_arp/arp_tables.h | 5 +- include/uapi/linux/netfilter_ipv4/ip_tables.h | 5 +- .../uapi/linux/netfilter_ipv6/ip6_tables.h | 5 +- net/ipv4/netfilter/arp_tables.c | 65 +++++----- net/ipv4/netfilter/ip_tables.c | 65 +++++----- net/ipv6/netfilter/ip6_tables.c | 65 +++++----- net/netfilter/x_tables.c | 79 ++++++------ 9 files changed, 192 insertions(+), 214 deletions(-) -- 2.47.2
