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From: Matthew Wilcox <willy@infradead.org>
To: Eric Biggers <ebiggers@kernel.org>
Cc: 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>,
	Alan Stern <stern@rowland.harvard.edu>,
	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 13:35:43 +0100
Message-ID: <20200717123543.GO12769@casper.infradead.org> (raw)
In-Reply-To: <20200717044427.68747-1-ebiggers@kernel.org>

On Thu, Jul 16, 2020 at 09:44:27PM -0700, Eric Biggers wrote:
> +The simplest implementation just uses a mutex and an 'inited' flag.

There's a perfectly good real word "initialised" / initialized.
https://chambers.co.uk/search/?query=inited&title=21st

> +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) {
> +			free_foo(p);
> +			/* pairs with successful cmpxchg_release() above */
> +			smp_load_acquire(&foo);
> +		}
> +		return 0;
> +	}
> +
> +Note that when the cmpxchg_release() fails due to another task already
> +having done it, a second smp_load_acquire() is required, since we still
> +need to acquire the data that the other task released.  You may be
> +tempted to upgrade cmpxchg_release() to cmpxchg() with the goal of it
> +acting as both ACQUIRE and RELEASE, but that doesn't work here because
> +cmpxchg() only guarantees memory ordering if it succeeds.
> +
> +Because of the above subtlety, the version with the mutex instead of
> +cmpxchg_release() should be preferred, except potentially in cases where
> +it is difficult to provide anything other than a global mutex and where
> +the one-time data is part of a frequently allocated structure.  In that
> +case, a global mutex might present scalability concerns.

There are concerns other than scalability where we might want to eliminate
the mutex.  For example, if (likely) alloc_foo() needs to allocate memory
and we would need foo to perform page writeback, then either we must
allocate foo using GFP_NOFS or do without the mutex, lest we deadlock
on this new mutex.

You might think this would argue for just using GFP_NOFS always, but
GFP_NOFS is a big hammer which forbids reclaiming from any filesystem,
whereas we might only need this foo to reclaim from a particular
filesystem.

  parent reply index

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 [this message]
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
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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