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

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Date:	Fri, 16 Oct 2015 18:11:05 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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

On 16 October 2015 at 17:04, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> 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.
>
> Well, arm64 might well need smp_mb__after_unlock_lock() to be non-empty.
> But I thought that it used a dmb in the spinlock code somewhere or
> another...

unlock+lock on arm64 is a full barrier, so, IIUC the semantics of
smp_mb__after_unlock_lock(), I don't think we need this to be
non-empty. Maybe redefining smp_mb__before_spinlock() to be smp_mb()
(rather than wmb) on arm64 if we need smp_mb__before_spinlock+lock to
act as a barrier for both reads and writes (see my other reply to
Peter).

-- 
Catalin
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