Messages in this thread | | | Date | Wed, 24 Nov 2010 10:53:17 +0800 | Subject | Re: [PATCH] arch/tile: fix rwlock so would-be write lockers don't block new readers | From | Cypher Wu <> |
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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. > > -- > Chris Metcalf, Tilera Corp. > http://www.tilera.com > >
If we count on that, should we make 'my_ticket_ = (val >> WR_NEXT_SHIFT) & WR_MASK;'?
-- Cyberman Wu http://www.meganovo.com
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