linux-kernel - Re: [PATCH] tools/memory-model: document the "one-time init" pattern

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Message-ID: <20200718010247.GC2183@sol.localdomain>
Date:   Fri, 17 Jul 2020 18:02:47 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...nel.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 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