[ovs-dev] [PATCH 07/18] lib/ovs-thread: Avoid atomic read in ovsthread_once_start().

[Jarno Rajahalme]

On Aug 28, 2014, at 9:42 AM, Ben Pfaff <blp at nicira.com> wrote:

> On Fri, Aug 22, 2014 at 01:58:18PM -0700, Jarno Rajahalme wrote:
>> Avoiding the atomic read may help if a function using
>> ovsthread_once_start() is ever called in a loop, as the new
>> 'maybe_not_done' can be kept in a register.  The atomic read will
>> still be done as long as 'maybe_not_done' is true.  Since
>> 'maybe_not_done' is not an atomic variable, whis may happen
>> indefinitely, but if a loop starts with 'maybe_not_done' as false, no
>> atomic read operations are needed.
>> 
>> Signed-off-by: Jarno Rajahalme <jrajahalme at nicira.com>
> 
> It seems reasonable to have a cheaper test like this, but I don't think
> that if we do that we need to also inline the more expensive test.  In
> other words, I think we could drop the call to
> ovsthread_once_is_done__() from ovsthread_once_start().  (After that we
> can move ovsthread_once_is_done__() into ovs-thread.c.)

How about getting rid of the atomic variable and just is a bool. On false negatives we first need to take the mutex anyway to have the thread wait for the initialization to complete, and after “once" is done, the first unnecessary (if any) mutex_lock will synchronize memory and after that the old ‘done’ value can not be cached any more?

I’ll post this in v2 for you to review.

  Jarno