linux-kernel - Re: Litmus test for question from Al Viro

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Message-ID: <20201005151639.GE376584@rowland.harvard.edu>
Date:   Mon, 5 Oct 2020 11:16:39 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Will Deacon <will@...nel.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>, parri.andrea@...il.com,
        peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
        dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        akiyks@...il.com, dlustig@...dia.com, joel@...lfernandes.org,
        viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: Litmus test for question from Al Viro

On Mon, Oct 05, 2020 at 04:13:13PM +0100, Will Deacon wrote:
> > The failure to recognize the dependency in P0 should be considered a 
> > combined limitation of the memory model and herd7.  It's not a simple 
> > mistake that can be fixed by a small rewrite of herd7; rather it's a 
> > deliberate choice we made based on herd7's inherent design.  We 
> > explicitly said that control dependencies extend only to the code in the 
> > branches of an "if" statement; anything beyond the end of the statement 
> > is not considered to be dependent.
> 
> Interesting. How does this interact with loops that are conditionally broken
> out of, e.g.  a relaxed cmpxchg() loop or an smp_cond_load_relaxed() call
> prior to a WRITE_ONCE()?

Heh --  We finesse this issue by not supporting loops at all!  :-)

Alan