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

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Date:	Mon, 23 May 2016 12:44:16 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Jason Low <jason.low2@....com>
Cc:	Peter Hurley <peter@...leysoftware.com>, jason.low2@...com,
	Waiman Long <waiman.long@....com>,
	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 Mon, 23 May 2016, 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  :)

Locking wise we should be covered with ->on_cpu as we're always under rcu_read_lock
(barrier, preempt_disable). But I'm not sure if this rule applies throughout the
scheduler, however, like it does in, say thread_group_cputime(). cpu_clock_sample()
(from posix timers) seems to mix and match being done under rcu. So ultimately I
think you're right.

Thanks,
Davidlohr