linux-kernel - Re: [RFC 1/3] mutex: In mutex_can_spin_on_owner(), return false if task need

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Message-ID: <20140115074844.GA3694@twins.programming.kicks-ass.net>
Date:	Wed, 15 Jan 2014 08:48:44 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	mingo@...hat.com, paulmck@...ux.vnet.ibm.com, Waiman.Long@...com,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	akpm@...ux-foundation.org, davidlohr@...com, hpa@...or.com,
	aswin@...com, scott.norton@...com
Subject: Re: [RFC 1/3] mutex: In mutex_can_spin_on_owner(), return false if
 task need_resched()

On Wed, Jan 15, 2014 at 08:44:20AM +0100, Peter Zijlstra wrote:
> On Tue, Jan 14, 2014 at 04:33:08PM -0800, Jason Low wrote:
> > The mutex_can_spin_on_owner() function should also return false if the
> > task needs to be rescheduled.
> > 
> 
> While I was staring at mutex_can_spin_on_owner(); don't we need this?
> 
>  kernel/locking/mutex.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 4dd6e4c219de..480d2f437964 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -214,8 +214,10 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock)
>  
>  	rcu_read_lock();
>  	owner = ACCESS_ONCE(lock->owner);
> -	if (owner)
> +	if (owner) {

That is, its an unmatched barrier, as mutex_set_owner() doesn't include
a barrier, and I don't think i needs to; but on alpha we still need this
read barrier to ensure we do not mess up this related load afaik.

Paul? can you explain an unpaired read_barrier_depends?

> +		smp_read_barrier_depends();
>  		retval = owner->on_cpu;
> +	}
>  	rcu_read_unlock();
>  	/*
>  	 * if lock->owner is not set, the mutex owner may have just acquired
--
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