Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE

[Peter Hurley]

On 05/18/2016 12:58 PM, Jason Low wrote:
> On Wed, 2016-05-18 at 14:29 -0400, Waiman Long wrote:
>> On 05/18/2016 01:21 PM, Jason Low wrote:
>>> On Wed, 2016-05-18 at 07:04 -0700, Davidlohr Bueso wrote:
>>>> On Tue, 17 May 2016, Waiman Long wrote:
>>>>
>>>>> Without using WRITE_ONCE(), the compiler can potentially break a
>>>>> write into multiple smaller ones (store tearing). So a read from the
>>>>> same data by another task concurrently may return a partial result.
>>>>> This can result in a kernel crash if the data is a memory address
>>>>> that is being dereferenced.
>>>>>
>>>>> This patch changes all write to rwsem->owner to use WRITE_ONCE()
>>>>> to make sure that store tearing will not happen. READ_ONCE() may
>>>>> not be needed for rwsem->owner as long as the value is only used for
>>>>> comparison and not dereferencing.
>>> It might be okay to leave out READ_ONCE() for reading rwsem->owner, but
>>> couldn't we include it to at least document that we're performing a
>>> "special" lockless read?
>>>
>>
>> Using READ_ONCE() does have a bit of cost as it limits compiler 
>> optimization. If we changes all access to rwsem->owner to READ_ONCE() 
>> and WRITE_ONCE(), we may as well change its type to volatile and be done 
>> with.
> 
> Right, although there are still places like the init function where
> WRITE_ONCE isn't necessary.

Which doesn't cost anything either.


>> I am not against doing that, but it feels a bit over-reach for me. 
>> On the other hand, we may define a do-nothing macro that designates the 
>> owner as a special variable for documentation purpose, but don't need 
>> protection at that particular call site.
> 
> It should be fine to use the standard READ_ONCE here, even if it's just
> for documentation, as it's probably not going to cost anything in
> practice. It would be better to avoid adding any special macros for this
> which may just add more complexity.

See, I don't understand this line of reasoning at all.

I read this as "it's ok to be non-optimal here where were spinning CPU
time but not ok to be non-optimal generally elsewhere where it's
way less important like at init time".

And by the way, it's not just "here" but _everywhere_.
What about reading ->on_cpu locklessly?

Sure it's a bool, but doesn't the "we need to document lockless access"
argument equally apply here?

Regards,
Peter Hurley