Re: [PATCH RFC LKMM 1/7] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

[Alan Stern]

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