Messages in this thread | | | Subject | Re: correct locking in softirq | From | Peter Zijlstra <> | Date | Fri, 27 Feb 2009 09:29:06 +0100 |
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On Fri, 2009-02-27 at 08:54 +0100, Giacomo wrote: > Good morning > > Harald Welte's "The journey of a packet through the Linux 2.6.10 > network stack" article says that packet travelling inside > linux kernel 2.6 (the receive / input part) runs in softirq context. > > Hooking with netfilter's hooks in a kernel module, i need to read for > each packet received a list of rules. > > Since in input and prerouting hooks the context is softirq (perhaps > also in forward?), I need some read lock > feature. > > I currently use RCU lists and, while reading lists I use > > READ > > read_lock_bh() > > together with list_for_each_rcu() > > When changing, or flushing, rules, I use > > WRITE > > spin_lock() + list_add_tail_rcu() (adding) > > or spin_lock() + list_for_each_entry() (for listing and then freeing > with list_del_rcu() and call_rcu() ) > > The question is: > > - is the read part above correct? - do I really need _bh()? or should > I use simply read_lock() ? > > Thanks in advance
rcu_read_lock() + call_rcu() are correct, even from softirq context, and mandatory if anything is exposed to anything other than softirq context.
rcu_read_lock_bh() + call_rcu_bh() is usable IFF the data is only ever used from softirq.
The distinction between the two RCU variants is that the _bh variant can have a slightly faster quiescent cycle.
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