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From: Eric Biggers <ebiggers@kernel.org>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: 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>,
	Alan Stern <stern@rowland.harvard.edu>,
	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
Date: Fri, 17 Jul 2020 17:58:57 -0700
Message-ID: <20200718005857.GB2183@sol.localdomain> (raw)
In-Reply-To: <20200717205340.GR7625@magnolia>

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

  reply index

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-17  4:44 Eric Biggers
2020-07-17  5:49 ` Sedat Dilek
2020-07-17 12:35 ` Matthew Wilcox
2020-07-17 14:26 ` Alan Stern
2020-07-17 17:47 ` Matthew Wilcox
2020-07-17 17:51   ` Alan Stern
2020-07-18  1:02     ` Eric Biggers
2020-07-27 12:51       ` Matthew Wilcox
2020-07-17 21:05   ` Darrick J. Wong
2020-07-18  0:44   ` Darrick J. Wong
2020-07-18  1:38   ` Eric Biggers
2020-07-18  2:13     ` Matthew Wilcox
2020-07-18  5:28       ` Eric Biggers
2020-07-18 14:35         ` Alan Stern
2020-07-20  2:07         ` Dave Chinner
2020-07-20  9:00           ` Peter Zijlstra
2020-07-27 15:17         ` Alan Stern
2020-07-27 15:28           ` Matthew Wilcox
2020-07-27 16:01             ` Paul E. McKenney
2020-07-27 16:31             ` Alan Stern
2020-07-27 16:59               ` Matthew Wilcox
2020-07-27 19:13                 ` Alan Stern
2020-07-17 20:53 ` Darrick J. Wong
2020-07-18  0:58   ` Eric Biggers [this message]
2020-07-18  1:25     ` Alan Stern
2020-07-18  1:40       ` Matthew Wilcox
2020-07-18  2:00       ` Dave Chinner
2020-07-18 14:21         ` Alan Stern
2020-07-18  2:00       ` Eric Biggers
2020-07-18  1:42 ` Dave Chinner
2020-07-18 14:08   ` Alan Stern
2020-07-20  1:33     ` Dave Chinner
2020-07-20 14:52       ` Alan Stern
2020-07-20 15:37         ` Darrick J. Wong
2020-07-20 15:39         ` Matthew Wilcox
2020-07-20 16:04           ` Paul E. McKenney
2020-07-20 16:48             ` peterz
2020-07-20 22:06               ` Paul E. McKenney
2020-07-20 16:12           ` Alan Stern

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