On Mon, Jul 18, 2016 at 2:48 PM, Thomas Graf <tg...@suug.ch> wrote: > On 07/18/16 at 01:39pm, Tom Herbert wrote: >> On Mon, Jul 18, 2016 at 11:10 AM, Thomas Graf <tg...@suug.ch> wrote: >> > I agree with that but I would like to keep the current per net_device >> > atomic properties. >> >> I don't see that see that there is any synchronization guarantees >> using xchg. For instance, if the pointer is set right after being read >> by a thread for one queue and right before being read by a thread for >> another queue, this could result in the old and new program running >> concurrently or old one running after new. If we need to synchronize >> the operation across all queues then sequence >> ifdown,modify-config,ifup will work. > > Right, there are no synchronization guarantees between threads and I > don't think that's needed. The guarantee that is provided is that if > I replace a BPF program, the replace either succeeds in which case > all packets have been either processed by the old or new program. Or > the replace failed in which case the old program was left intact and > all packets are still going through the old program. > > This is a nice atomic replacement principle which would be nice to > preserve.
Sure, if replace operation fails then old program should remain in place. But xchg can't fail, so it seems like part is just giving a false sense of security that program replacement is somehow synchronized across queues. Tom
