> On 11/23/2010 9:53 PM, Cypher Wu wrote:
>> 2010/11/24 Chris Metcalf <cmetcalf@tilera.com>:
>>> On 11/22/2010 8:36 PM, Cypher Wu wrote:
>>>> Say, if core A try to write_lock() rwlock and current_ticket_ is 0 and
>>>> it write next_ticket_ to 1, when it processing the lock, core B try to
>>>> write_lock() again and write next_ticket_ to 2, then when A
>>>> write_unlock() it seen that (current_ticket_+1) is not equal to
>>>> next_ticket_, so it increment current_ticket_, and core B get the
>>>> lock. If core A try write_lock again before core B write_unlock, it
>>>> will increment next_ticket_ to 3. And so on.
>>>> This may rarely happened, I've tested it yesterday for several hours
>>>> it goes very well under pressure.
>>> This should be OK when it happens (other than starving out the readers, but
>>> that was the decision made by doing a ticket lock in the first place).
>>> Even if we wrap around 255 back to zero on the tickets, the ticket queue
>>> will work correctly. The key is not to need more than 256 concurrent write
>>> lock waiters, which we don't.
>> If we count on that, should we make 'my_ticket_ = (val >>
>> WR_NEXT_SHIFT) & WR_MASK;'
>
> No, it's OK. As the comment for the declaration of "my_ticket_" says, the
> trailing underscore reminds us that the high bits are garbage, and when we
> use the value, we do the mask: "((my_ticket_ - curr_) & WR_MASK)". It
> turned out doing the mask here made the most sense from a code-generation
> point of view, partly just because of the possibility of the counter wrapping.
>
> --
> Chris Metcalf, Tilera Corp.
>
http://www.tilera.com
>
>
