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Subject: Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock
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From: Tim Chen <tim.c.chen@linux.intel.com>
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Date: Mon, 24 Jun 2013 13:17:45 -0700
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To: Peter Hurley <peter@hurleysoftware.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>, Alex Shi <alex.shi@intel.com>, Michel Lespinasse <walken@google.com>, Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>, Andrea Arcangeli <aarcange@redhat.com>, Andi Kleen <andi@firstfloor.org>, Matthew R Wilcox <matthew.r.wilcox@intel.com>, Dave Hansen <dave.hansen@intel.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Rik van Riel <riel@redhat.com>, linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>

On Mon, 2013-06-24 at 14:49 -0400, Peter Hurley wrote:
> On 06/24/2013 01:11 PM, Tim Chen wrote:
> > On Sun, 2013-06-23 at 13:03 -0700, Davidlohr Bueso wrote:
> >> On Sat, 2013-06-22 at 03:57 -0400, Peter Hurley wrote:
> >>> On 06/21/2013 07:51 PM, Tim Chen wrote:
> >>>>
> >>>> +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> >>>> +{
> >>>> +	int retval = true;
> >>>> +
> >>>> +	/* Spin only if active writer running */
> >>>> +	if (!sem->owner)
> >>>> +		return false;
> >>>> +
> >>>> +	rcu_read_lock();
> >>>> +	if (sem->owner)
> >>>> +		retval = sem->owner->on_cpu;
> >>>                            ^^^^^^^^^^^^^^^^^^
> >>>
> >>> Why is this a safe dereference? Could not another cpu have just
> >>> dropped the sem (and thus set sem->owner to NULL and oops)?
> >>>
> >
> > The rcu read lock should protect against sem->owner being NULL.
> 
> It doesn't.
> 
> Here's the comment from mutex_spin_on_owner():
> 
>    /*
>     * Look out! "owner" is an entirely speculative pointer
>     * access and not reliable.
>     */

On second thought, I agree with you.  I should change this to
something like

	int retval = true;
	task_struct *sem_owner;

	/* Spin only if active writer running */
	if (!sem->owner)
		return false;

	rcu_read_lock();
	sem_owner = sem->owner;
	if (sem_owner)
		retval = sem_owner->on_cpu;

Thanks.

Tim


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