Re: [RFC] Cancellable MCS spinlock rework

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

On Wed, 2014-07-02 at 10:30 -0700, Jason Low wrote:
> On Wed, 2014-07-02 at 19:23 +0200, Peter Zijlstra wrote:
> > On Wed, Jul 02, 2014 at 09:59:16AM -0700, Jason Low wrote:
> > > 
> > > Why I converted pointers to atomic_t?
> > > 
> > > This would avoid the potentially racy ACCESS_ONCE stores + cmpxchg while
> > > also using less overhead, since atomic_t is often only 32 bits while
> > > pointers could be 64 bits.
> > 
> > So no real good reason.. The ACCESS_ONCE stores + cmpxchg stuff is
> > likely broken all over the place, and 'fixing' this one place doesn't
> > cure the problem.
> 
> Right, fixing the ACCESS_ONCE + cmpxchg and avoiding the architecture
> workarounds for optimistic spinning was just a nice side effect.
> 
> Would potentially reducing the size of the rw semaphore structure by 32
> bits (for all architectures using optimistic spinning) be a nice
> benefit?

And due to padding, the additional modification below reduces the
size of struct rw_semaphore by 64 bits on my machine  :)


 struct rw_semaphore {
 	long count;
-	raw_spinlock_t wait_lock;
 	struct list_head wait_list;
+	raw_spinlock_t wait_lock;
 #ifdef CONFIG_SMP
+	struct optimistic_spin_tail osq; /* spinner MCS lock */
 	/*
 	 * Write owner. Used as a speculative check to see
 	 * if the owner is running on the cpu.
 	 */
 	struct task_struct *owner;
-	struct optimistic_spin_tail osq; /* spinner MCS lock */
 #endif
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map	dep_map;