Re: [RFC PATCH] riscv/locking: Strengthen spin_lock() and spin_unlock()

[Will Deacon <will.deacon@arm.com>]

On Thu, Feb 22, 2018 at 11:47:57AM -0800, Daniel Lustig wrote:
> On 2/22/2018 10:27 AM, Peter Zijlstra wrote:
> > On Thu, Feb 22, 2018 at 10:13:17AM -0800, Paul E. McKenney wrote:
> >> So we have something that is not all that rare in the Linux kernel
> >> community, namely two conflicting more-or-less concurrent changes.
> >> This clearly needs to be resolved, either by us not strengthening the
> >> Linux-kernel memory model in the way we were planning to or by you
> >> strengthening RISC-V to be no weaker than PowerPC for these sorts of
> >> externally viewed release-acquire situations.
> >>
> >> Other thoughts?
> > 
> > Like said in the other email, I would _much_ prefer to not go weaker
> > than PPC, I find that PPC is already painfully weak at times.
> 
> Sure, and RISC-V could make this work too by using RCsc instructions
> and/or by using lightweight fences instead.  It just wasn't clear at
> first whether smp_load_acquire() and smp_store_release() were RCpc,
> RCsc, or something else, and hence whether RISC-V would actually need
> to use something stronger than pure RCpc there.  Likewise for
> spin_unlock()/spin_lock() and everywhere else this comes up.
> 
> As Paul's email in the other thread observed, RCpc seems to be
> OK for smp_load_acquire()/smp_store_release() at least according
> to the current LKMM herd spec.  Unlock/lock are stronger already
> I guess.  But if there's an active proposal to strengthen them all
> to something stricter than pure RCpc, then that's good to know.
> 
> My understanding from earlier discussions is that ARM has no plans
> to use their own RCpc instruction for smp_load_acquire() instead
> of their RCsc instructions.  Is that still true?  If they were to
> use the RCpc load there, that would cause them to have the same
> problem we're discussing here, right?  Just checking.

Agreed. No plans to use the LDAPR instruction in Linux.

Will