Hi Pablo, 2017-03-24 20:17 GMT+08:00 Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>: >> --- a/net/netfilter/nfnetlink_cttimeout.c >> +++ b/net/netfilter/nfnetlink_cttimeout.c >> @@ -646,8 +646,8 @@ static void __exit cttimeout_exit(void) >> #ifdef CONFIG_NF_CONNTRACK_TIMEOUT >> RCU_INIT_POINTER(nf_ct_timeout_find_get_hook, NULL); >> RCU_INIT_POINTER(nf_ct_timeout_put_hook, NULL); >> + synchronize_rcu(); >> #endif /* CONFIG_NF_CONNTRACK_TIMEOUT */ >> - rcu_barrier(); > > cttimeout relies on kfree_rcu(). > > Are you sure we don't need this? > > According to: > > https://www.kernel.org/doc/Documentation/RCU/rcubarrier.txt > > "We could try placing a synchronize_rcu() in the module-exit code path, > but this is not sufficient." > > Then: > > "Please note that rcu_barrier() does -not- imply synchronize_rcu(), in > particular, if there are no RCU callbacks queued anywhere, > rcu_barrier() is within its rights to return immediately, without > waiting for a grace period to elapse." This is because we use kfree_rcu to free the cttimeout objects. So I think rcu_barrier() is not needed anymore. Quoted from https://lwn.net/Articles/433493/ : "And kfree_rcu() is also help for unloadable modules, kfree_rcu() does not queue any function which belong to the module, so a rcu_barrier() can be avoid when module exit." Also from commit 9ab1544eb419 ("rcu: introduce kfree_rcu()"): "Many rcu callbacks functions just call kfree() on the base structure. These functions are trivial, but their size adds up, and furthermore when they are used in a kernel module, that module must invoke the high-latency rcu_barrier() function at module-unload time." -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
