[Sheepdog] [PATCH] zookeeper: switch gcc atomic builtins to atomic helpers

[Yunkai Zhang]

On Tue, May 8, 2012 at 10:41 PM, Christoph Hellwig <hch at infradead.org> wrote:
> On Tue, May 08, 2012 at 08:40:34PM +0800, Yunkai Zhang wrote:
>> > ? ? ? ?if (!uatomic_read(&zk_notify_blocked) &&
>> > ? ? ? ? ? ? uatomic_read(&nr_zk_levents)) {
>> > ? ? ? ? ? ? ? ?nr_levents = uatomic_sub_return(&nr_zk_levents, 1) + 1;
>> >
>> > I guess he means the possible race between uatomic_read() and
>> > uatomic_sub_return() on the same value.
>>
>> I have explained - when we are trying to subtract nr_zk_levents, it
>> will be added by watcher() in another thread.
>
> I still don't really understand the problem.  If we need this to be
> atomic (and I think we do) we'd need something like
> atomic_dec_if_positive in the Linux Kernel - unfortunately urcu does not
> seem to provide an equivalent.

Yes, if gcc supports atomic_dec_if_positive(), I like to use it, but it doesn't.

It works well now, I don't know what you worry about?

>
> Normally you'd use a proper lock for this sort of concurrency issue,
> if you need something lockless you'd want a relatively complicated
> producer/consumer queue like the one in include/linux/llist.h in the
> Linux Kernel.



-- 
Yunkai Zhang
Work at Taobao