Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier

Davidlohr Bueso
2016-06-15T18:56:11Z

On Wed, 15 Jun 2016, Peter Zijlstra wrote:

>On Wed, Jun 15, 2016 at 11:27:24AM -0700, Davidlohr Bueso wrote:
>> On Wed, 15 Jun 2016, Peter Zijlstra wrote:
>>
>> >In any case, its fairly simple to cure, just add
>> >smp_acquire__after_ctrl_dep() at the end. If we bail because
>> >need_resched() we don't need the acquire I think.
>>
>> I was just considering this for your smp_cond_acquire/smp_cond_load_acquire
>
>Right, so that need_resched break makes that a bit awkward. Not to
>mention the cpu_relaxed() vs cpu_relaxed_lowlatency() difference.

Oh sure, I was merely refering to the ordering semantics, not the calls
themselves -- although at some point, as archs begin to port locking/core
optimizations, we _will_ need the variants for dealing with '_lowlatency'.

>
>> rework, so yeah I guess an smp_acquire__after_ctrl_dep would be a nice
>> compromise.
>>
>> However, I was always under the impression that races with node->locked were
>> rather harmless (as indicated in the mentioned commit) -- which is why ->locked
>> are simple load/stores, with the exception of the unqueueing -- but yeah, that's
>> not even paired.
>
>Yeah, see a few patches further in this series, where he guards a
>variables with the osq_lock.

*sigh*