Re: [patch 05/15] Generic Mutex Subsystem, mutex-core.patch

[Steven Rostedt <rostedt@kihontech.com>]

On Mon, 2005-12-19 at 00:00 -0500, Steven Rostedt wrote:
> On Mon, 2005-12-19 at 02:37 +0100, Ingo Molnar wrote:
> > +static inline int
> > +__mutex_lock_common(struct mutex *lock, struct mutex_waiter *waiter,
> > +                   struct thread_info *ti, struct task_struct *task,
> > +                   unsigned long *flags, unsigned long task_state
> > __IP_DECL__)
> > +{
> > +       unsigned int old_val;
> > +
> > +       debug_lock_irqsave(&debug_lock, *flags, ti);
> > +       DEBUG_WARN_ON(lock->magic != lock);
> > +
> 
> How expensive is the xchg?  Since __mutex_lock_common is called even
> when it's going to wake up. Maybe it might be more efficient to add
> something like:

But add:

#ifdef MUTEX_LOCKLESS_FASTPATH
> 
>           if (atomic_cmpxchg(&lock->count, 1, 0) {
>               debug_set_owner(lock, ti __IP__);
>               debug_unlock_irqrestore(&debug_lock, *flags, ti);
>               return 1;
> 	  }
#endif

-- Steve

> 
> This way we save the overhead of grabbing another spinlock, adding the
> task to the wait_list and changing it's state.
> 
> 
> > +       spin_lock(&lock->wait_lock);
> > +       __add_waiter(lock, waiter, ti, task __IP__);
> > +       set_task_state(task, task_state);
> > +
> > +       /*
> > +        * Lets try to take the lock again - this is needed even if
> > +        * we get here for the first time (shortly after failing to
> > +        * acquire the lock), to make sure that we get a wakeup once
> > +        * it's unlocked. Later on this is the operation that gives
> > +        * us the lock. We need to xchg it to -1, so that when we
> > +        * release the lock, we properly wake up other waiters!
> > +        */
> > +       old_val = atomic_xchg(&lock->count, -1);
> > +
> > +       if (old_val == 1) {
> 
> -- Steve
> 
> 
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-- 
Steven Rostedt
Senior Programmer
Kihon Technologies
(607)786-4830