Re: Unlock-lock questions and the Linux Kernel Memory Model

[Boqun Feng <boqun.feng@gmail.com>]

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On Wed, Nov 29, 2017 at 02:44:37PM -0500, Alan Stern wrote:
> On Wed, 29 Nov 2017, Daniel Lustig wrote:
> 
> > While we're here, let me ask about another test which isn't directly
> > about unlock/lock but which is still somewhat related to this
> > discussion:
> > 
> > "MP+wmb+xchg-acq" (or some such)
> > 
> > {}
> > 
> > P0(int *x, int *y)
> > {
> >         WRITE_ONCE(*x, 1);
> >         smp_wmb();
> >         WRITE_ONCE(*y, 1);
> > }
> > 
> > P1(int *x, int *y)
> > {
> >         r1 = atomic_xchg_relaxed(y, 2);
> >         r2 = smp_load_acquire(y);
> >         r3 = READ_ONCE(*x);
> > }
> > 
> > exists (1:r1=1 /\ 1:r2=2 /\ 1:r3=0)
> > 
> > C/C++ would call the atomic_xchg_relaxed part of a release sequence
> > and hence would forbid this outcome.
> > 
> > x86 and Power would forbid this.  ARM forbids this via a special-case
> > rule in the memory model, ordering atomics with later load-acquires.
> > 
> > RISC-V, however, wouldn't forbid this by default using RCpc or RCsc
> > atomics for smp_load_acquire().  It's an "fri; rfi" type of pattern,
> > because xchg doesn't have an inherent internal data dependency.
> > 
> > If the Linux memory model is going to forbid this outcome, then
> > RISC-V would either need to use fences instead, or maybe we'd need to
> > add a special rule to our memory model similarly.  This is one detail
> > where RISC-V is still actively deciding what to do.
> > 
> > Have you all thought about this test before?  Any idea which way you
> > are leaning regarding the outcome above?
> 
> Good questions.  Currently the LKMM allows this, and I think it should
> because xchg doesn't have a dependency from its read to its write.
> 
> On the other hand, herd isn't careful enough in the way it implements 
> internal dependencies for RMW operations.  If we change 
> atomic_xchg_relaxed(y, 2) to atomic_inc(y) and remove r1 from the test:
> 
> C MP+wmb+inc-acq
> 
> {}
> 
> P0(int *x, int *y)
> {
>         WRITE_ONCE(*x, 1);
>         smp_wmb();
>         WRITE_ONCE(*y, 1);
> }
> 
> P1(int *x, int *y)
> {
>         atomic_inc(y);
>         r2 = smp_load_acquire(y);
>         r3 = READ_ONCE(*x);
> }
> 
> exists (1:r2=2 /\ 1:r3=0)
> 
> then the test _should_ be forbidden, but it isn't -- herd doesn't
> realize that all atomic RMW operations other than xchg must have a
> dependency (either data or control) between their internal read and
> write.
> 
> (Although the smp_load_acquire is allowed to execute before the write 
> part of the atomic_inc, it cannot execute before the read part.  I 
> think a similar argument applies even on ARM.)
> 

But in case of AMOs, which directly send the addition request to memory
controller, so there wouldn't be any read part or even write part of the
atomic_inc() executed by CPU. Would this be allowed then?

Regards,
Boqun

> Luc, consider this a bug report.  :-)
> 
> Alan
> 

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