Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

[Linus Torvalds]

On Fri, 9 Jan 2009, Peter Zijlstra wrote:
> 
> On that note:
> 
> Index: linux-2.6/kernel/mutex.c
> ===================================================================
> --- linux-2.6.orig/kernel/mutex.c
> +++ linux-2.6/kernel/mutex.c
> @@ -220,7 +220,9 @@ __mutex_lock_common(struct mutex *lock, 
>  		__set_task_state(task, state);
>  
>  		/* didnt get the lock, go to sleep: */
> +		preempt_disable();
>  		spin_unlock_mutex(&lock->wait_lock, flags);
> +		preempt_enable_no_resched();
>  		schedule();

Yes. I think this is a generic issue independently of the whole adaptive 
thing.

In fact, I think wee could make the mutex code use explicit preemption and 
then the __raw spinlocks to make this more obvious. Because now there's a 
hidden "preempt_enable()" in that spin_unlock_mutex, and anybody looking 
at the code and not realizing it is going to just say "Whaa? Who is this 
crazy Peter Zijlstra guy, and what drugs is he on? I want me some!".

Because your patch really doesn't make much sense unless you know how 
spinlocks work, and if you _do_ know how spinlocks work, you go "eww, 
that's doing extra preemption crud in order to just disable the 
_automatic_ preemption crud".

			Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/