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

[Peter Hurley <peter@hurleysoftware.com>]

On 06/24/2013 04:17 PM, Tim Chen wrote:
> 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;
>

Our emails passed each other.

Also, I haven't given a lot of thought to if preemption must be disabled
before calling rwsem_can_spin_on_owner(). If so, wouldn't you just drop
rwsem_can_spin_on_owner() (because the conditions tested in the loop are
equivalent)?

Regards,
Peter Hurley