Messages in this thread | | | Date | Fri, 11 Jan 2019 06:08:10 -0800 | From | "Paul E. McKenney" <> | Subject | Re: seqcount usage in xt_replace_table() |
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On Fri, Jan 11, 2019 at 09:34:11AM +0100, Peter Zijlstra wrote: > On Thu, Jan 10, 2019 at 11:29:20PM +0100, Florian Westphal wrote: > > Peter Zijlstra <peterz@infradead.org> wrote: > > > Would using synchronize_rcu() not also mean you can get rid of that > > > xt_write_recseq*() stuff entirely? > > > > No, because those are used to synchronize with cpus that read > > the ruleset counters, see > > > > net/ipv4/netfilter/ip_tables.c:get_counters(). > > Ah, bummer :/ > > > > Anyway, synchronize_rcu() can also take a little while, but I don't > > > think anywere near 30 seconds. > > > > Ok, I think in that case it would be best to just replace the > > recseq value sampling with smp_mb + synchronize_rcu plus a comment > > that explains why its done. > > synchronize_rcu() implies smp_mb() on all CPUs.
Yes, it does, but in the case of idle CPUs, the smp_mb() calls are only required to follow any pre-existing RCU read-side critical section on the one hand an precede any RCU read-side critical section completing after the synchronize_rcu() on the other.
To do more would mean waking up idle CPUs, which does not make the battery-powered guys happy. ;-)
Thanx, Paul
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