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Date:   Fri, 17 Jul 2020 19:00:09 -0700
From:   Eric Biggers <ebiggers@kernel.org>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     "Darrick J. Wong" <darrick.wong@oracle.com>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        "Paul E . McKenney" <paulmck@kernel.org>,
        linux-fsdevel@vger.kernel.org, Akira Yokosawa <akiyks@gmail.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        Dave Chinner <david@fromorbit.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Nicholas Piggin <npiggin@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>
Subject: Re: [PATCH] tools/memory-model: document the "one-time init" pattern
Message-ID: <20200718020009.GE2183@sol.localdomain>
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 <20200718005857.GB2183@sol.localdomain>
 <20200718012555.GA1168834@rowland.harvard.edu>
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On Fri, Jul 17, 2020 at 09:25:55PM -0400, Alan Stern wrote:
> On Fri, Jul 17, 2020 at 05:58:57PM -0700, Eric Biggers wrote:
> > 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.
> 
> Perhaps something like this:
> 
> 	Specifically, if the only way to reach the initialized memory 
> 	involves dereferencing the pointer itself then READ_ONCE() is 
> 	sufficient.  This is because there will be an address dependency 
> 	between reading the pointer and accessing the memory, which will 
> 	ensure proper ordering.  But if some of the initialized memory 
> 	is reachable some other way (for example, if it is global or 
> 	static data) then there need not be an address dependency, 
> 	merely a control dependency (checking whether the pointer is 
> 	non-NULL).  Control dependencies do not always ensure ordering 
> 	-- certainly not for reads, and depending on the compiler, 
> 	possibly not for some writes -- and therefore a load-acquire is 
> 	necessary.
> 
> Perhaps this is more wordy than you want, but it does get the important 
> ideas across.
> 

How about:

	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 the only way to reach the initialized
	memory involves dereferencing the pointer itself, then the data
	dependency barrier provided by READ_ONCE() is sufficient.  However, if
	some of the initialized memory is reachable some other way (for example,
	if it is global or static data) then there need not be an address
	dependency, merely a control dependency (checking whether the pointer is
	non-NULL).  READ_ONCE() is *not* sufficient in that case.

	The optimization of replacing smp_load_acquire() with READ_ONCE() is
	discouraged for nontrivial data structures, since it can be difficult to
	determine if it is correct.  In particular, for complex data structures
	the correctness of the READ_ONCE() optimization may depend on internal
	implementation details of other kernel subsystems.