Re: [PATCH net-next 2/2] net: initialize hashrnd in flow_dissector with net_get_random_once

On Wed, Oct 23, 2013 at 06:58:34AM -0700, Eric Dumazet wrote:

On Wed, 2013-10-23 at 13:12 +0200, Hannes Frederic Sowa wrote:

quoted

We also can defer the initialization of hashrnd in flow_dissector
to its first use. Since net_get_random_once is irqsave now we don't
have to audit the call paths if one of this functions get called by an
interrupt handler.

Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Hannes Frederic Sowa <redacted>
---

This really works well if CONFIG_JUMP_LABEL=y

I am afraid some arches do not really have this.

This is the problem with all users of net_get_random_once.

If an architecture does not have JUMP_LABEL or gcc does not provide
CC_HAVE_ASM_GOTO we have an unlikely branch in the code path and branch
to the epilogue of the function to generate the hash for the first
time. After that it's "just" a likely atomic boolean test.

I guess it is not that important to have a good secret hash key here as
this is solely used to dispatch the packets to cpus and does not store
anything in tables.

Greetings,

  Hannes