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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Alan Stern <stern@rowland.harvard.edu>,
Eric Biggers <ebiggers@kernel.org>,
linux-kernel@vger.kernel.org, linux-arch@vger.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>,
"Darrick J . Wong" <darrick.wong@oracle.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: Mon, 27 Jul 2020 09:01:11 -0700 [thread overview]
Message-ID: <20200727160111.GH9247@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200727152827.GM23808@casper.infradead.org>
On Mon, Jul 27, 2020 at 04:28:27PM +0100, Matthew Wilcox wrote:
> On Mon, Jul 27, 2020 at 11:17:46AM -0400, Alan Stern wrote:
> > Given a type "T", an object x of type pointer-to-T, and a function
> > "func" that takes various arguments and returns a pointer-to-T, the
> > accepted API for calling func once would be to create once_func() as
> > follows:
> >
> > T *once_func(T **ppt, args...)
> > {
> > static DEFINE_MUTEX(mut);
> > T *p;
> >
> > p = smp_load_acquire(ppt); /* Mild optimization */
> > if (p)
> > return p;
> >
> > mutex_lock(mut);
> > p = smp_load_acquire(ppt);
> > if (!p) {
> > p = func(args...);
> > if (!IS_ERR_OR_NULL(p))
> > smp_store_release(ppt, p);
> > }
> > mutex_unlock(mut);
> > return p;
> > }
> >
> > Users then would have to call once_func(&x, args...) and check the
> > result. Different x objects would constitute different "once"
> > domains.
> [...]
> > In fact, the only drawback I can think of is that because this relies
> > on a single mutex for all the different possible x's, it might lead to
> > locking conflicts (if func had to call once_func() recursively, for
> > example). In most reasonable situations such conflicts would not
> > arise.
>
> Another drawback for this approach relative to my get_foo() approach
> upthread is that, because we don't have compiler support, there's no
> enforcement that accesses to 'x' go through once_func(). My approach
> wraps accesses in a deliberately-opaque struct so you have to write
> some really ugly code to get at the raw value, and it's just easier to
> call get_foo().
Could ACCESS_PRIVATE() help in this case? This relies on sparse rather
than the compiler, but some of the testing services do run sparse
regularly.
Thanx, Paul
next prev parent reply other threads:[~2020-07-27 16:01 UTC|newest]
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 [this message]
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
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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