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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 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?

Alan

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