Re: [RFC] Cancellable MCS spinlock rework

[Jason Low <jason.low2@hp.com>]

On Thu, 2014-07-03 at 16:35 -0400, Waiman Long wrote:

> I do see a point in reducing the size of the rwsem structure. However, I 
> don't quite understand the point of converting pointers in the 
> optimistic_spin_queue structure to atomic_t. The structure is cacheline 
> aligned and there is no saving in size. Converting them to atomic_t does 
> have a bit of additional overhead of converting the encoded cpu number 
> back to the actual pointer.
> 
> So my suggestion is to just change what is stored in the mutex and rwsem 
> structure to atomic_t, but keep the pointers in the 
> optimistic_spin_queue structure.

Peter, would you prefer going with the above?

If we were to keep the pointers to the next and prev nodes in the struct
optimistic_spin_queue instead of converting them to atomic_t to store
their cpu #, we'd still need to keep track of the cpu #. In the unqueue
phase of osq_lock, we might have to reload prev = node->prev which we
then may cmpxchg() it with the lock tail.

The method we can think of so far would be to add a regular int variable
to optimistic_spin_queue and initialize it to the CPU #, during the time
we also initialize node->locked and node->next at the beginning of
osq_lock. The cost wouldn't be much of an issue since
optimistic_spin_queue is cache aligned.