Messages in this thread | | | Date | Fri, 17 Jun 2016 09:46:59 -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, 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.
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