Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier
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- To: Waiman Long <waiman.long@xxxxxxx>
- Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier
- From: Davidlohr Bueso <dave@xxxxxxxxxxxx>
- Date: Fri, 17 Jun 2016 09:29:26 -0700
- Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, x86@xxxxxxxxxx, linux-alpha@xxxxxxxxxxxxxxx, linux-ia64@xxxxxxxxxxxxxxx, linux-s390@xxxxxxxxxxxxxxx, linux-arch@xxxxxxxxxxxxxxx, Jason Low <jason.low2@xxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Scott J Norton <scott.norton@xxxxxxx>, Douglas Hatch <doug.hatch@xxxxxxx>
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- References: <1465944489-43440-1-git-send-email-Waiman.Long@hpe.com> <1465944489-43440-2-git-send-email-Waiman.Long@hpe.com> <20160615165659.GC2094@linux-80c1.suse> <20160615171250.GO30921@twins.programming.kicks-ass.net> <20160615182724.GD2094@linux-80c1.suse> <20160615184007.GW30921@twins.programming.kicks-ass.net> <20160617011155.GA14591@linux-80c1.suse> <576408F7.8020901@hpe.com>
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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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