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

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Message-ID: <20200717174750.GQ12769@casper.infradead.org>
Date:   Fri, 17 Jul 2020 18:47:50 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     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>,
        Alan Stern <stern@...land.harvard.edu>,
        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 Thu, Jul 16, 2020 at 09:44:27PM -0700, Eric Biggers wrote:
> +If that doesn't apply, you'll have to implement one-time init yourself.
> +
> +The simplest implementation just uses a mutex and an 'inited' flag.
> +This implementation should be used where feasible:

I think some syntactic sugar should make it feasible for normal people
to implement the most efficient version of this just like they use locks.

> +For the single-pointer case, a further optimized implementation
> +eliminates the mutex and instead uses compare-and-exchange:
> +
> +	static struct foo *foo;
> +
> +	int init_foo_if_needed(void)
> +	{
> +		struct foo *p;
> +
> +		/* pairs with successful cmpxchg_release() below */
> +		if (smp_load_acquire(&foo))
> +			return 0;
> +
> +		p = alloc_foo();
> +		if (!p)
> +			return -ENOMEM;
> +
> +		/* 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?

> +			free_foo(p);
> +			/* pairs with successful cmpxchg_release() above */
> +			smp_load_acquire(&foo);
> +		}
> +		return 0;
> +	}

How about something like this ...

once.h:

static struct init_once_pointer {
	void *p;
};

static inline void *once_get(struct init_once_pointer *oncep)
{ ... }

static inline bool once_store(struct init_once_pointer *oncep, void *p)
{ ... }

--- foo.c ---

struct foo *get_foo(gfp_t gfp)
{
	static struct init_once_pointer my_foo;
	struct foo *foop;

	foop = once_get(&my_foo);
	if (foop)
		return foop;

	foop = alloc_foo(gfp);
	if (!once_store(&my_foo, foop)) {
		free_foo(foop);
		foop = once_get(&my_foo);
	}

	return foop;
}

Any kernel programmer should be able to handle that pattern.  And no mutex!