linux-kernel - Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock acquisition

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Message-ID: <51C894C3.4040407@hurleysoftware.com>
Date:	Mon, 24 Jun 2013 14:49:39 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
CC:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Alex Shi <alex.shi@...el.com>,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock acquisition

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.
    */

Regards,
Peter Hurley

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