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

Manfred Spraul
2016-08-31T18:32:28Z

On 08/31/2016 06:40 PM, Will Deacon wrote:
>
> I'm struggling with this example. We have these locks:
>
> &sem->lock
> &sma->sem_base[0...sma->sem_nsems].lock
> &sma->sem_perm.lock
>
> a condition variable:
>
> sma->complex_mode
>
> and a new barrier:
>
> smp_mb__after_spin_lock()
>
> For simplicity, we can make sma->sem_nsems == 1, and have &sma->sem_base[0]
> be &sem->lock in the example above.
Correct.
> &sma->sem_perm.lock seems to be
> irrelevant.
Correct.
> 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.

So I had assumed:

CPUm:

LOCK(x)
smp_mb(); /* at least for PPC, therefore with arch override */
RyAcq=0

CPUn:

Wy=1
smp_mb(); /* at least for archs where UNLOCK_WAIT is RxAcq */
UNLOCK_WAIT(x)
smp_rmb(); /* not required anymore, was required when UNLOCK_WAIT was Rx */

--
Manfred