linux-kernel - Re: [RFC] Bridging the gap between the Linux Kernel Memory Consistency Model (LKMM) and C11/C++11 atomics

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Message-ID: <3f740262-821b-4f39-8f1a-c6d02253986c@paulmck-laptop>
Date:   Tue, 4 Jul 2023 14:25:09 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Olivier Dion <odion@...icios.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rnk@...gle.com, Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, gcc@....gnu.org, llvm@...ts.linux.dev
Subject: Re: [RFC] Bridging the gap between the Linux Kernel Memory
 Consistency Model (LKMM) and C11/C++11 atomics

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