linux-kernel - Re: [PATCH] tools/memory-model: document the "one-time init" pattern

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Message-ID: <20200718005857.GB2183@sol.localdomain>
Date:   Fri, 17 Jul 2020 17:58:57 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...nel.org>,
        linux-fsdevel@...r.kernel.org, Akira Yokosawa <akiyks@...il.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Dave Chinner <david@...morbit.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] tools/memory-model: document the "one-time init" pattern

On Fri, Jul 17, 2020 at 01:53:40PM -0700, Darrick J. Wong wrote:
> > +There are also cases in which the smp_load_acquire() can be replaced by
> > +the more lightweight READ_ONCE().  (smp_store_release() is still
> > +required.)  Specifically, if all initialized memory is transitively
> > +reachable from the pointer itself, then there is no control dependency
> 
> I don't quite understand what "transitively reachable from the pointer
> itself" means?  Does that describe the situation where all the objects
> reachable through the object that the global struct foo pointer points
> at are /only/ reachable via that global pointer?
> 

The intent is that "transitively reachable" means that all initialized memory
can be reached by dereferencing the pointer in some way, e.g. p->a->b[5]->c.

It could also be the case that allocating the object initializes some global or
static data, which isn't reachable in that way.  Access to that data would then
be a control dependency, which a data dependency barrier wouldn't work for.

It's possible I misunderstood something.  (Note the next paragraph does say that
using READ_ONCE() is discouraged, exactly for this reason -- it can be hard to
tell whether it's correct.)  Suggestions of what to write here are appreciated.

- Eric