[ovs-dev] [PATCH v4 04/28] datapath: backport: net: add dst_cache support

[pravin shelar]

On Thu, Jul 7, 2016 at 6:04 PM, Jesse Gross <jesse at kernel.org> wrote:
> On Thu, Jul 7, 2016 at 5:23 PM, Pravin B Shelar <pshelar at ovn.org> wrote:
>> This backports dst-cache implementation from upstream implementation.
>>
>>     commit 911362c70df5b766c243dc297fadeaced786ffd8
>>     Author: Paolo Abeni <pabeni at redhat.com>
>>
>>     net: add dst_cache support
>>     This patch add a generic, lockless dst cache implementation.
>>     The need for lock is avoided updating the dst cache fields
>>     only in per cpu scope, and requiring that the cache manipulation
>>     functions are invoked with the local bh disabled.
>>
>>     The refresh_ts and reset_ts fields are used to ensure the cache
>>     consistency in case of cuncurrent cache update (dst_cache_set*) and
>>     reset operation (dst_cache_reset).
>>
>>     Consider the following scenario:
>>
>>     CPU1:                                       CPU2:
>>       <cache lookup with emtpy cache: it fails>
>>       <get dst via uncached route lookup>
>>                                                 <related configuration changes>
>>                                                 dst_cache_reset()
>>       dst_cache_set()
>>
>>     The dst entry set passed to dst_cache_set() should not be used
>>     for later dst cache lookup, because it's obtained using old
>>     configuration values.
>>
>>     Since the refresh_ts is updated only on dst_cache lookup, the
>>     cached value in the above scenario will be discarded on the next
>>     lookup.
>>
>>     Signed-off-by: Paolo Abeni <pabeni at redhat.com>
>>     Suggested-and-acked-by: Hannes Frederic Sowa <hannes at stressinduktion.org>
>>     Signed-off-by: David S. Miller <davem at davemloft.net>
>>
>> Signed-off-by: Pravin B Shelar <pshelar at ovn.org>
>> Acked-by: Jesse Gross <jesse at kernel.org>
>
> I saw in the big Geneve/VXLAN backport patch that USE_UPSTREAM_TUNNEL
> becomes dependent on having the dst cache. That's fine but I couldn't
> find anywhere that prevents the reverse - continuing to use our
> version of the dst cache when we have upstream tunnels (the functions
> here look like unconditional replacements). It seems like that will
> create a problem in a couple weeks when 4.7 is released.

As discussed I am planing on doing it later when I add support for new
kernel. I can not test it without compiling it with new kernel.