linux-kernel - Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE

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Date:	Tue, 24 May 2016 21:25:46 -0400
From:	Waiman Long <waiman.long@....com>
To:	Jason Low <jason.low2@....com>
CC:	Peter Hurley <peter@...leysoftware.com>, <jason.low2@...com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by
 WRITE_ONCE

On 05/23/2016 02:46 PM, Jason Low wrote:
> On Sat, 2016-05-21 at 09:04 -0700, Peter Hurley wrote:
>> On 05/18/2016 12:58 PM, Jason Low wrote:
>>> 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".
> So I think there is a difference between using it during init time and
> using it here where we're spinning. During init time, initializing the
> owner field locklessly is normal. No other thread should be concurrently
> be writing to the field, since the structure is just getting
> initialized, so there are no surprises there.
>
> Our access of the owner field in this function is special in that we're
> using a bit of "lockless magic" to read and write to a field that gets
> concurrently accessed without any serialization. Since we're not taking
> the wait_lock in a scenario where we'd normally would take a lock, it
> would be good to have this documented.
>
>> And by the way, it's not just "here" but _everywhere_.
>> What about reading ->on_cpu locklessly?
> Sure, we could also use READ_ONCE when reading ->on_cpu  :)
>

As on_cpu is just a boolean, load tearing isn't really a problem. You 
either see the bit 0 set or not, but not something in between (not a 
qbit)  :-)

Cheers,
Longman