[ovs-dev] [PATCH 06/18] lib/ovs-thread: Use atomic_count.

Jarno Rajahalme jrajahalme at nicira.com
Fri Aug 29 19:50:24 UTC 2014

On Aug 29, 2014, at 9:36 AM, Jarno Rajahalme <jrajahalme at nicira.com> wrote:

> On Aug 28, 2014, at 9:32 AM, Ben Pfaff <blp at nicira.com> wrote:
>> On Fri, Aug 22, 2014 at 01:58:17PM -0700, Jarno Rajahalme wrote:
>>> barrier->count is used as a simple counter and is not expected the
>>> synchronize the state of any other variable, so we can use atomic_count,
>>> which uses relaxed atomics.
>>> Ditto for the 'next_id' within ovsthread_wrapper().
>>> Signed-off-by: Jarno Rajahalme <jrajahalme at nicira.com>
>> I would have thought that ovs_barrier_block() would include a fairly
>> strong barrier--at least an acquire-release barrier?--but it doesn't
>> seem to necessarily include one at all now.
> Yes, it would seem reasonable to expect that the effects of all participating threads would be visible on return from ovs_barrier_block(). Even after this change the seq_read() does that (due to ovs_mutex_lock() and ovs_mutex_unlock(), which have acquire and release semantics, respectively). However, seq_read() could be implemented without using a mutex, so it would be better to not rely on it.

I looked into this a bit more and I now think seq makes no sense without acquire-release semantics (which it already implements), so I plan to document it and then rely on it. Let’s see what you think.


> I’ll post a v2 on this to address this, thanks!
>  Jarno

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