Messages in this thread | | | Date | Fri, 27 Feb 2009 08:59:42 -0800 | From | "Paul E. McKenney" <> | Subject | Re: correct locking in softirq |
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On Fri, Feb 27, 2009 at 09:29:06AM +0100, Peter Zijlstra wrote: > 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.
If "softirq" also includes sections of local_bh_disable()ed code in process context, also including irq-disabled code, agreed!
> The distinction between the two RCU variants is that the _bh variant can > have a slightly faster quiescent cycle.
Especially when under heavy interrupt/softirq load. If a given CPU is totally consumed handling interrupts and softirqs in a non-CONFIG_PREEMPT_RT kernel, then the call_rcu() variant might never invoke its callback, while the call_rcu_bh() variant would still be able to do so in a timely fashion. The _bh() variant was inspired by simulate DoS attacks, work by Robert Olsson and Dipankar Sarma.
Thanx, Paul
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