linux-kernel - Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner field

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Date:	Sun, 15 May 2016 10:47:47 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner
 field

On 05/13/2016 01:58 PM, Peter Hurley wrote:
> On 05/13/2016 08:07 AM, Peter Zijlstra wrote:
>> On Wed, May 11, 2016 at 03:04:20PM -0700, Peter Hurley wrote:
>>>> +	return !rwsem_is_reader_owned(READ_ONCE(sem->owner));
>>> It doesn't make sense to force reload sem->owner here; if sem->owner
>>> is not being reloaded then the loop above will execute forever.
>>>
>>> Arguably, this check should be bumped out to the optimistic spin and
>>> reload/check the owner there?
>>>
>> Note that barrier() and READ_ONCE() have overlapping but not identical
>> results and the combined use actually makes sense here.
>>
>> Yes, a barrier() anywhere in the loop will force a reload of the
>> variable, _however_ it doesn't force that reload to not suffer from
>> load tearing.
>>
>> Using volatile also forces a reload, but also ensures the load cannot
>> be torn IFF it is of machine word side and naturally aligned.
>>
>> So while the READ_ONCE() here is pointless for forcing the reload;
>> that's already ensured, we still need to make sure the load isn't torn.
> If load tearing a naturally aligned pointer is a real code generation
> possibility then the rcu list code is broken too (which loads ->next
> directly; cf. list_for_each_entry_rcu()&  list_for_each_entry_lockless()).
>
> For 4.4, Paul added READ_ONCE() checks for list_empty() et al, but iirc
> that had to do with control dependencies and not load tearing.
>
> OTOH, this patch might actually produce store-tearing:
>
> +static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
> +{
> +	/*
> +	 * We check the owner value first to make sure that we will only
> +	 * do a write to the rwsem cacheline when it is really necessary
> +	 * to minimize cacheline contention.
> +	 */
> +	if (sem->owner != RWSEM_READER_OWNED)
> +		sem->owner = RWSEM_READER_OWNED;
> +}
>
>
> Regards,
> Peter Hurley

While load tearing in the argument to rwsem_is_reader_owned() isn't an 
issue as the wrong decision won't do any harm. Store tearing as 
identified above can be a problem. I will fix that even though the the 
chance of compiling generating store tearing code is really small.

Cheers,
Longman