linux-kernel - Re: [PATCH 4/5] locking/rwsem: Avoid deceiving lock spinners

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Message-ID: <20150131092921.GB32343@twins.programming.kicks-ass.net>
Date:	Sat, 31 Jan 2015 10:29:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Michel Lespinasse <walken@...gle.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 4/5] locking/rwsem: Avoid deceiving lock spinners

On Fri, Jan 30, 2015 at 01:14:26AM -0800, Davidlohr Bueso wrote:
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -337,21 +337,30 @@ static inline bool owner_running(struct rw_semaphore *sem,
>  static noinline
>  bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
>  {
> +	long count;
> +
>  	rcu_read_lock();
>  	while (owner_running(sem, owner)) {
> +		/* abort spinning when need_resched */
> +		if (need_resched()) {
> +			rcu_read_unlock();
> +			return false;
> +		}
>  
>  		cpu_relax_lowlatency();
>  	}
>  	rcu_read_unlock();
>  
> +	if (READ_ONCE(sem->owner))
> +		return true; /* new owner, continue spinning */
> +

Same concern as Tim; also the mutex code seems to terminate the spin
when owner changes. And I think we want to have writers behave similar
to mutexes, no?

Does it make sense to change things to allow owner changes from NULL,
but not to NULL?

>  	/*
> +	 * When the owner is not set, the lock could be free or
> +	 * held by readers. Check the counter to verify the
> +	 * state.
>  	 */
> -	return sem->owner == NULL;
> +	count = READ_ONCE(sem->count);
> +	return (count == 0 || count == RWSEM_WAITING_BIAS);
>  }
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