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

From: Davidlohr Bueso
Date: Fri Jun 17 2016 - 12:47:18 EST

On Fri, 17 Jun 2016, Davidlohr Bueso wrote:

On Fri, 17 Jun 2016, Waiman Long wrote:

On 06/16/2016 09:11 PM, Davidlohr Bueso wrote:
On Wed, 15 Jun 2016, Peter Zijlstra wrote:

Yeah, see a few patches further in this series, where he guards a
variables with the osq_lock.

So one problem I have with all this is that if we are hardening osq_lock/unlock()
because of some future use that is specific to rwsems, then we will immediately
be hurting mutexes for no good reason.

I am going to change it to use smp_acquire__after_ctrl_dep() as suggested by PeterZ. Is that a good enough compromise? I have also changed the xchg in the unlock side to xchg_release which could help performance in some archs. The thing is when developers see the name osq_lock/osq_unlock, they will naturally assume the proper barrriers are provided which is not currently the case.

Oh, from your discussions with Boqun, I was under the impression that ->locked
was now going to be properly ordered in all cases now, which is why
I worry about mutexes.

Anyway, the change won't affect x86, it is probably ARM or PPC that may have an impact.

Yes, that xchg() won't affect x86, but adding an smp_store_release(node->locked, 1)
or such will obviously.

nm this last part, you're right, x86 smp_store_release is a nop.