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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Message-ID: <20180831182845.GA4673@andrea>
Date:   Fri, 31 Aug 2018 20:28:46 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        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, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com
Subject: Re: [PATCH RFC LKMM 1/7] tools/memory-model: Add extra ordering for
 locks and remove it for ordinary release/acquire

> > Yes, it's true that implementing locks with atomic_cmpxchg_acquire 
> > should be correct on all existing architectures.  And Paul has invited 
> > a patch to modify the LKMM accordingly.  If you feel that such a change 
> > would be a useful enhancement to the LKMM's applicability, please write 
> > it.
> 
> Yes, please! That would be the "RmW" discussion which Andrea partially
> quoted earlier on, so getting that going independently from this patch
> sounds like a great idea to me.

That was indeed one of the proposal we discussed.  As you recalled, that
proposal only covered RmWs load-acquire (and ordinary store-release); in
particular, I realized that comments such as:

  "The atomic_cond_read_acquire() call above has provided the
   necessary acquire semantics required for locking."

   [from kernel/locking/qspinlock.c]

(for example)  would still _not have "generic validity" _if we added the
above po-unlock-rf-lock-po term... (which, again, makes me somehow uncon-
fortable);  Would to have _all_ the acquire be admissible for you?

  Andrea


> 
> Cheers,
> 
> Will