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

[Andrea Parri <andrea.parri@amarulasolutions.com>]

On Thu, Jul 12, 2018 at 01:52:49PM +0200, Andrea Parri wrote:
> On Thu, Jul 12, 2018 at 09:40:40AM +0200, Peter Zijlstra wrote:
> > 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.
> 
> Indeed, what I was suggesting above is a weaking of the current model
> (and I agree: I wouldn't say that bugs in this context are nice  ;-).
> 
> These changes would affect a specific area: (IMO,) the examples we've
> been considering here aren't for the faint-hearted  ;-) and as Daniel
> already suggested, everything would again be "nice and neat", if this
> was all about locking/if every thread used lock-synchronization.
> 
> 
> > 
> > >   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.
> 
> You mean: "when paired with a po-earlier release to the same memory
> location", right?  I am afraid that neither arm64 nor riscv current
> implementations would ensure "(r1 == 1 && r2 == 0) forbidden" if we
> removed the po-earlier spin_unlock()...
> 
> AFAICT, the current implementation would work with that release: as
> you remarked above, arm64 release->acquire is SC; riscv has an rw,w
> fence in its spin_unlock() (hence an w,w fence between the stores),
> or it could have a .tso fence ...
> 
> But again, these are stuble patterns, and my guess is that several/
> most kernel developers really won't care about such guarantees (and
> if some will do, they'll have the tools to figure out what they can
> actually rely on ...)
> 
> OTOH (as I pointed out earlier) the strengthening we're configuring
> will prevent some arch. (riscv being just the example of today!) to
> go "full RCsc", and this will inevitably "complicate" both the LKMM

"full RCpc"

  Andrea


> and the reviewing process of related changes (atomics, locking, ...;
> c.f., this debate), apparently, just because you  ;-) want to "care"
> about these guarantees.
> 
> Not yet convinced ...  :/
> 
>   Andrea
> 
> 
> > 
> > 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.