Re: [PATCH 1/4] spinlock: Document memory barrier rules

Manfred Spraul
2016-09-01T11:04:33Z

Hi,

On 09/01/2016 10:44 AM, Peter Zijlstra wrote:
> On Wed, Aug 31, 2016 at 08:32:18PM +0200, Manfred Spraul wrote:
>> On 08/31/2016 06:40 PM, Will Deacon wrote:
>>> The litmus test then looks a bit like:
>>>
>>> CPUm:
>>>
>>> LOCK(x)
>>> smp_mb();
>>> RyAcq=0
>>>
>>>
>>> CPUn:
>>>
>>> Wy=1
>>> smp_mb();
>>> UNLOCK_WAIT(x)
>> Correct.
>>> which I think can be simplified to:
>>>
>>>
>>> LOCK(x)
>> I thought that here a barrier is required, because Ry=0 could be before
>> store of the lock.
>>> Ry=0
>> RyAcq instead of Ry would required due to the unlock at the end of the
>> critical section
>> CpuN: <...>
>> WyRelease=0
>> for the litmus test irrelevant.
>>> Wy=1
>>> smp_mb(); // Note that this is implied by spin_unlock_wait on PPC and arm64
>>> LOCK(x) // spin_unlock_wait behaves like lock; unlock
>>> UNLOCK(x)
>>> [I've removed a bunch of barriers here, that I don't think are necessary
>>> for the guarantees you're after]
>>>
>>> and the question is "Can both CPUs proceed?".
>>>
>>> Looking at the above, then I don't think that they can. Whilst CPUm can
>>> indeed speculate the Ry=0 before successfully taking the lock, if CPUn
>>> observes CPUm's read, then it must also observe the lock being held wrt
>>> the spin_lock API. That is because a successful LOCK operation by CPUn
>>> would force CPUm to replay its LL/SC loop and therefore discard its
>>> speculation of y.
>>>
>>> What am I missing? The code snippet seems to have too many barriers to me!
>> spin_unlock_wait() is not necessarily lock()+unlock().
>> It can be a simple Rx, or now RxAcq.
> Can be, normally, yes. But on power and arm64, the only architectures on
> which the ACQUIRE is 'funny' they do the 'pointless' ll/sc cycle in
> spin_unlock_wait() to 'fix' things.
>
> So for both power and arm64, you can in fact model spin_unlock_wait()
> as LOCK+UNLOCK.
Is this consensus?

If I understand it right, the rules are:
1. spin_unlock_wait() must behave like spin_lock();spin_unlock();
2. spin_is_locked() must behave like spin_trylock() ? spin_unlock(),TRUE
: FALSE
3. the ACQUIRE during spin_lock applies to the lock load, not to the store.

sem.c and nf_conntrack.c need only rule 1 now, but I would document the
rest as well, ok?

I'll update the patches.

--
Manfred