On Tue, Jan 08, 2019 at 11:37:46PM +0100, Florian Westphal wrote: > Anatol Pomozov <anatol.pomo...@gmail.com> wrote: > > Or maybe xt_replace_table() can be enhanced? When I hear that > > something waits until an event happens on all CPUs I think about > > wait_event() function. Would it be better for xt_replace_table() to > > introduce an atomic counter that is decremented by CPUs, and the main > > CPU waits until the counter gets zero? > > That would mean placing an additional atomic op into the > iptables evaluation path (ipt_do_table and friends). >
For: /* * Ensure contents of newinfo are visible before assigning to * private. */ smp_wmb(); table->private = newinfo; we have: smp_store_release(&table->private, newinfo); But what store does that second smp_wmb() order against? The comment: /* make sure all cpus see new ->private value */ smp_wmb(); makes no sense what so ever, no smp_*() barrier can provide such guarantees. > Only alternative I see that might work is synchronize_rcu (the > _do_table functions are called with rcu read lock held). > > I guess current scheme is cheaper though. Is performance a concern in this path? There is no comment justifying this 'creative' stuff.