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SubjectRe: [PATCH RFC LKMM 1/7] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
On Fri, Sep 07, 2018 at 12:00:19PM -0400, Alan Stern wrote:
> On Thu, 6 Sep 2018, Andrea Parri wrote:
>
> > > Have you noticed any part of the generic code that relies on ordinary
> > > acquire-release (rather than atomic RMW acquire-release) in order to
> > > implement locking constructs?
> >
> > There are several places in code where the "lock-acquire" seems to be
> > provided by an atomic_cond_read_acquire/smp_cond_load_acquire: I have
> > mentioned one in qspinlock in this thread; qrwlock and mcs_spinlock
> > provide other examples (grep for the primitives...).
> >
> > As long as we don't consider these primitive as RMW (which would seem
> > odd...) or as acquire for which "most people expect strong ordering"
> > (see above), these provides other examples for the _gap_ I mentioned.
>
> Okay, now I understand your objection. It does appear that on RISC-V,
> if nowhere else, the current implementations of qspinlock, qrwlock,
> etc. will not provide "RCtso" ordering.
>
> The discussions surrounding this topic have been so lengthy and
> confusing that I have lost track of any comments Palmer or Daniel may
> have made concerning this potential problem.
>
> One possible resolution would be to define smp_cond_load_acquire()
> specially on RISC-V so that it provided the same ordering guarantees as
> RMW-acquire. (Plus adding a comment in the asm-generic/barrier.h
> pointing out the necessity for the stronger guarantee on all
> architectures.)
>
> Another would be to replace the usages of atomic/smp_cond_load_acquire
> in the locking constructs with a new function that would otherwise be
> the same but would provide the ordering guarantee we want.
>
> Do you think either of these would be an adequate fix?

I didn't think RISC-V used qspinlock or qrwlock, so I'm not sure there's
actually anything to fix, is there?

Will

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