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Date: Wed, 28 Mar 2007 09:04:59 +0200
From: Nick Piggin <npiggin@suse.de>
To: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Ravikiran G Thirumalai <kiran@scalex86.org>, Ingo Molnar <mingo@elte.hu>,
       Nikita Danilov <nikita@clusterfs.com>,
       Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org
Subject: Re: [patch] queued spinlocks (i386)
Message-ID: <20070328070459.GC12508@wotan.suse.de>
References: <20070325155407.GA497@tv-sign.ru>
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On Sun, Mar 25, 2007 at 07:54:07PM +0400, Oleg Nesterov wrote:
> I am sorry for being completely off-topic, but I've been wondering for the
> long time...
> 
> What if we replace raw_spinlock_t.slock with "struct task_struct *owner" ?
> 
> 	void _spin_lock(spinlock_t *lock)
> 	{
> 		struct task_struct *owner;
> 
> 		for (;;) {
> 			preempt_disable();
> 			if (likely(_raw_spin_trylock(lock)))
> 				break;
> 			preempt_enable();
> 
> 			while (!spin_can_lock(lock)) {
> 				rcu_read_lock();
> 				owner = lock->owner;
> 				if (owner && current->prio < owner->prio &&
> 				    !test_tsk_thread_flag(owner, TIF_NEED_RESCHED))
> 					set_tsk_thread_flag(owner, TIF_NEED_RESCHED);
> 				rcu_read_unlock();
> 				cpu_relax();
> 			}
> 		}
> 
> 		lock->owner = current;
> 	}
> 
> 	void _spin_unlock(spinlock_t *lock)
> 	{
> 		lock->owner = NULL;
> 		_raw_spin_unlock(lock);
> 		preempt_enable();
> 	}
> 
> Now we don't need need_lockbreak(lock), need_resched() is enough, and we take
> ->prio into consideration.
> 
> Makes sense? Or stupid?

Well with my queued spinlocks, all that lockbreak stuff can just come out
of the spin_lock, break_lock out of the spinlock structure, and
need_lockbreak just becomes (lock->qhead - lock->qtail > 1).