[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20200727160111.GH9247@paulmck-ThinkPad-P72>
Date: Mon, 27 Jul 2020 09:01:11 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: Alan Stern <stern@...land.harvard.edu>,
Eric Biggers <ebiggers@...nel.org>,
linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
linux-fsdevel@...r.kernel.org, Akira Yokosawa <akiyks@...il.com>,
Andrea Parri <parri.andrea@...il.com>,
Boqun Feng <boqun.feng@...il.com>,
Daniel Lustig <dlustig@...dia.com>,
"Darrick J . Wong" <darrick.wong@...cle.com>,
Dave Chinner <david@...morbit.com>,
David Howells <dhowells@...hat.com>,
Jade Alglave <j.alglave@....ac.uk>,
Luc Maranget <luc.maranget@...ia.fr>,
Nicholas Piggin <npiggin@...il.com>,
Peter Zijlstra <peterz@...radead.org>,
Will Deacon <will@...nel.org>
Subject: Re: [PATCH] tools/memory-model: document the "one-time init" pattern
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
Powered by blists - more mailing lists