linux-kernel - Re: [RFC,PATCH] mutex: mutex_is

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Date:	Wed, 04 Nov 2009 18:19:35 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC,PATCH] mutex: mutex_is_owner() helper

Ingo Molnar a écrit :

> To make sure this does not extend mutexes to be used a recursive 
> mutexes, mind naming it more clearly, like debug_mutex_is_owned(), and 
> adding a comment that says that this shouldnt be taken?
> 
> Also, it's somewhat imprecise: on !SMP && !DEBUG_MUTEXES we might return 
> a false '1'. Which happens to work for the rtnl usecase - but might not 
> in other cases.
>

Sure, we can chose another name, but what do you mean by a false '1' ?

1 means mutex is locked and that we could not check ownership.
(best effort, ie same imprecise result than mutex_is_locked())


BTW, I was thinking of a mutex_yield() implementation, but could not
cook it without hard thinking, maybe you already have some nice implementation ?

We have some uses of "mutex_unlock();mutex_lock();" things that are
not working nicely because current thread immediately takes again mutex.


a true mutex_yield() would force current thread to go at the end of wait_list.

int mutex_yield(struct mutex *lock)
{
	unsigned long flags;

	// OK to test list without locking
	if (list_empty(&lock->wait_list))
		return 0;

	spin_lock_mutex(&lock->wait_lock, flags);
	if (!list_empty(&lock->wait_list)) {
		atomic_xchg(&lock->count, 1);// free mutex
		list_add_tail(&waiter.list, &lock->wait_list);//insert me at tail of wait_list
		wake head of wait_list
		__mutex_lock_common_condadd(mutex, TASK_UNINTERRUPTIBLE, DONT_ADD_TAIL, ...);
	} else {
		spin_unlock_mutex(&lock->wait_lock, flags);
	}
	return 1;
}


Or maybe we should try something less complex (slowpath anyway)

int mutex_yield(struct mutex *lock)
{
	int ret = 0;

	if (mutex_needbreak(lock) || should_resched()) {
		mutex_unlock(lock); 
		__cond_resched();
		mutex_lock(lock);
		ret = 1;
	}
	return ret;
}

Thanks
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