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

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Date:   Fri, 7 Sep 2018 17:09:50 +0100
From:   Will Deacon <will.deacon@....com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        Andrea Parri <parri.andrea@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Kernel development list <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, mingo@...nel.org, peterz@...radead.org,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>, akiyks@...il.com,
        Palmer Dabbelt <palmer@...ive.com>,
        Daniel Lustig <dlustig@...dia.com>
Subject: Re: [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