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Date:   Tue, 25 Feb 2020 16:42:03 -0500 (EST)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Luc Maranget <luc.maranget@inria.fr>
cc:     Boqun Feng <boqun.feng@gmail.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: More on reader-writer locks
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On Tue, 25 Feb 2020, Luc Maranget wrote:

> Hi,
> 
> As far as I can remember I have implemented atomic_add_unless in herd7.

Luc, have you considered whether we can use atomic_add_unless and
cmpxchg to implement reader-writer locks in the LKMM?  I don't think we
can handle them the same way we handle ordinary locks now.

Let's say that a lock variable holds 0 if it is unlocked, -1 if it is 
write-locked, and a positive value if it is read-locked (the value is 
the number of read locks currently in effect).  Then operations like 
write_lock, write_trylock, and so on could all be modeled using 
variants of atomic_add_unless, atomic_dec, and cmpxchg.

But will that work if the reads-from relation is computed by the cat 
code in lock.cat?  I suspect it won't.

How would you approach this problem?

Alan