Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

Peter Zijlstra
2018-07-12T07:40:56Z

On Wed, Jul 11, 2018 at 02:34:21PM +0200, Andrea Parri wrote:
> Simplicity is the eye of the beholder. From my POV (LKMM maintainer), the
> simplest solution would be to get rid of rfi-rel-acq and unlock-rf-lock-po
> (or its analogous in v3) all together:

<snip>

> Among other things, this would immediately:
>
> 1) Enable RISC-V to use their .aq/.rl annotations _without_ having to
> "worry" about tso or release/acquire fences; IOW, this will permit
> a partial revert of:
>
> 0123f4d76ca6 ("riscv/spinlock: Strengthen implementations with fences")
> 5ce6c1f3535f ("riscv/atomic: Strengthen implementations with fences")

But I feel this goes in the wrong direction. This weakens the effective
memory model, where I feel we should strengthen it.

Currently PowerPC is the weakest here, and the above RISC-V changes
(reverts) would make RISC-V weaker still.

Any any effective weakening makes me very uncomfortable -- who knows
what will come apart this time. This memory ordering stuff causes
horrible subtle bugs at best.

> 2) Resolve the above mentioned controversy (the inconsistency between
> - locking operations and atomic RMWs on one side, and their actual
> implementation in generic code on the other), thus enabling the use
> of LKMM _and_ its tools for the analysis/reviewing of the latter.

This is a good point; so lets see if there is something we can do to
strengthen the model so it all works again.

And I think if we raise atomic*_acquire() to require TSO (but ideally
raise it to RCsc) we're there.

The TSO archs have RCpc load-acquire and store-release, but fully
ordered atomics. Most of the other archs have smp_mb() everything, with
the exception of PPC, ARM64 and now RISC-V.

PPC has the RCpc TSO fence LWSYNC, ARM64 has the RCsc
load-acquire/store-release. And RISC-V has a gazillion of options IIRC.

So ideally atomic*_acquire() + smp_store_release() will be RCsc, and is
with the notable exception of PPC, and ideally RISC-V would be RCsc
here. But at the very least it should not be weaker than PPC.

By increasing atomic*_acquire() to TSO we also immediately get the
proposed:

P0()
{
WRITE_ONCE(X, 1);
spin_unlock(&s);
spin_lock(&s);
WRITE_ONCE(Y, 1);
}

P1()
{
r1 = READ_ONCE(Y);
smp_rmb();
r2 = READ_ONCE(X);
}

behaviour under discussion; because the spin_lock() will imply the TSO
ordering.

And note that this retains regular RCpc ACQUIRE for smp_load_acquire()
and associated primitives -- as they have had since their introduction
not too long ago.