Messages in this thread | | | Date | Fri, 17 Jun 2016 09:29:26 -0700 | From | Davidlohr Bueso <> | Subject | Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier |
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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.
Thanks, Davidlohr
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