linux-kernel - Re: Q: schedule() and implied barriers on arm64

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Date:	Fri, 16 Oct 2015 17:55:35 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Will Deacon <will.deacon@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: Q: schedule() and implied barriers on arm64

I'll try to reply in Will's absence, though I gave up trying to
understand these threads long time ago ;).

On 16 October 2015 at 17:16, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Oct 16, 2015 at 09:04:22AM -0700, Paul E. McKenney wrote:
>> On Fri, Oct 16, 2015 at 05:18:30PM +0200, Peter Zijlstra wrote:
>> > If so, however, I suspect AARGH64 is borken and would need (just like
>> > PPC):
>> >
>> > #define smp_mb__before_spinlock()   smp_mb()
>> >
>> > The problem is that schedule() (when a NO-OP) does:
>> >
>> >     smp_mb__before_spinlock();
>> >     LOCK rq->lock
>> >
>> >     clear_bit()
>> >
>> >     UNLOCK rq->lock
>> >
>> > And nothing there implies a full barrier on AARGH64, since
>> > smp_mb__before_spinlock() defaults to WMB, LOCK is an "ldaxr" or
>> > load-acquire, UNLOCK is "stlrh" or store-release and clear_bit() isn't
>> > anything.
>> >
>> > Pretty much every other arch has LOCK implying a full barrier, either
>> > because its strongly ordered or because it needs one for the ACQUIRE
>> > semantics.
>>
>> But I thought that it used a dmb in the spinlock code somewhere or
>> another...
>
> arm does, arm64 not so much.

arm64 indeed does not have a dmb after spin_lock, it only has a
load-acquire. So with the default smp_mb__before_spinlock() +
spin_lock we have:

  smp_wmb()
  loop
    load-acquire
    store

So (I think) this guarantees that any writes before wmb+lock would be
visible before any reads _and_ writes after wmb+lock. However, the
ordering with reads before wmb+lock is not guaranteed.

-- 
Catalin
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