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From: Eric Biggers <ebiggers@kernel.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Matthew Wilcox <willy@infradead.org>,
	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>,
	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: Fri, 17 Jul 2020 18:02:47 -0700	[thread overview]
Message-ID: <20200718010247.GC2183@sol.localdomain> (raw)
In-Reply-To: <20200717175138.GB1156312@rowland.harvard.edu>

On Fri, Jul 17, 2020 at 01:51:38PM -0400, Alan Stern wrote:
> On Fri, Jul 17, 2020 at 06:47:50PM +0100, Matthew Wilcox wrote:
> > On Thu, Jul 16, 2020 at 09:44:27PM -0700, Eric Biggers wrote:
> ...
> > > +		/* on success, pairs with smp_load_acquire() above and below */
> > > +		if (cmpxchg_release(&foo, NULL, p) != NULL) {
> > 
> > Why do we have cmpxchg_release() anyway?  Under what circumstances is
> > cmpxchg() useful _without_ having release semantics?
> 
> To answer just the last question: cmpxchg() is useful for lock 
> acquisition, in which case it needs to have acquire semantics rather 
> than release semantics.
> 

To clarify, there are 4 versions of cmpxchg:

	cmpxchg(): does ACQUIRE and RELEASE (on success)
	cmpxchg_acquire(): does ACQUIRE only (on success)
	cmpxchg_release(): does RELEASE only (on success)
	cmpxchg_relaxed(): no barriers

The problem here is that here we need RELEASE on success and ACQUIRE on failure.
But no version guarantees any barrier on failure.

So as far as I can tell, the best we can do is use cmpxchg_release() (or
cmpxchg() which would be stronger but unnecessary), followed by a separate
ACQUIRE on failure.

- Eric

  reply	other threads:[~2020-07-18  1:02 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-17  4:44 [PATCH] tools/memory-model: document the "one-time init" pattern 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 [this message]
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
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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