Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 4 Jul 2023 14:25:09 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Olivier Dion <odion@efficios.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        rnk@google.com, Andrea Parri <parri.andrea@gmail.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org, gcc@gcc.gnu.org, llvm@lists.linux.dev
Subject: Re: [RFC] Bridging the gap between the Linux Kernel Memory
 Consistency Model (LKMM) and C11/C++11 atomics
Message-ID: <3f740262-821b-4f39-8f1a-c6d02253986c@paulmck-laptop>
Reply-To: paulmck@kernel.org
References: <87ttukdcow.fsf@laura>
 <feb9c2c0-24ce-40bf-a865-5898ffad3005@rowland.harvard.edu>
 <87ilazd278.fsf@laura>
 <bcdd09ec-b98f-42d6-b59d-64db953076f6@rowland.harvard.edu>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <bcdd09ec-b98f-42d6-b59d-64db953076f6@rowland.harvard.edu>
Precedence: bulk

On Tue, Jul 04, 2023 at 04:25:45PM -0400, Alan Stern wrote:
> On Tue, Jul 04, 2023 at 01:19:23PM -0400, Olivier Dion wrote:

[ . . . ]

> > I am puzzled by this.  Initialization of a shared variable does not need
> > to be atomic until its publication.  Could you expand on this?
> 
> In the kernel, I believe it sometimes happens that an atomic variable 
> may be published before it is initialized.  (If that's wrong, Paul or 
> Peter can correct me.)  But since this doesn't apply to the situations 
> you're concerned with, you can forget I mentioned it.

Both use cases exist.

A global atomic is implicitly published at compile time.  If the desired
initial value is not known until multiple threads are running, then it
is necessary to be careful.  Hence double-check locking and its various
replacements.  (Clearly, if you can determine the initial value before
going multithreaded, life is simpler.)

And dynamically allocated or on-stack storage is the other case, where
there is a point in time when the storage is private even after multiple
threads are running.

Or am I missing the point?

							Thanx, Paul