Re: [PATCH -v11][RFC] mutex: implement adaptive spinning

Nick Piggin
2009-01-14T17:18:00Z

On Wed, Jan 14, 2009 at 06:00:36PM +0100, Peter Zijlstra wrote:
> Full series, including changelogs available at:
>
> http://programming.kicks-ass.net/kernel-patches/mutex-adaptive-spin/
>
> and should shortly appear in a git tree near Ingo :-)

Linus is going to take a wholesale conversion of mutexes to adaptive
mutexes? He's gone soft. I put on my asbestos underwear for no reason,
then.

> @@ -173,21 +237,21 @@ __mutex_lock_common(struct mutex *lock,
> spin_unlock_mutex(&lock->wait_lock, flags);
>
> debug_mutex_free_waiter(&waiter);
> + preempt_enable();
> return -EINTR;
> }
> __set_task_state(task, state);
>
> /* didnt get the lock, go to sleep: */
> spin_unlock_mutex(&lock->wait_lock, flags);
> - schedule();
> + __schedule();

Why does this need to do a preempt-disabled schedule? After we schedule
away, the next task can do arbitrary things or reschedule itself, so if
we have not anticipated such a condition here, then I can't see what
__schedule protects. At least a comment is in order?

Pity to add the call overhead to schedule just for this case. BTW.
__schedule shouldn't need to be asmlinkage?

> spin_lock_mutex(&lock->wait_lock, flags);
> }
>