linux-kernel - Re: [PATCH v4] lib/dlock-list: Scale dlock_lists

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Date:   Thu, 9 Nov 2017 18:30:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        Andreas Dilger <adilger@...ger.ca>, Jan Kara <jack@...e.cz>,
        Waiman Long <longman@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jan Kara <jack@...e.com>,
        Jeff Layton <jlayton@...chiereds.net>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Andi Kleen <andi@...stfloor.org>,
        Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH v4] lib/dlock-list: Scale dlock_lists_empty()

On Thu, Nov 09, 2017 at 09:24:08AM -0800, Davidlohr Bueso wrote:
> On Wed, 08 Nov 2017, Boqun Feng wrote:
> > Or worse:
> > 
> > 	 * CPU0				    CPU1
> > 	 * dlock_list_add()                 dlock_lists_empty()
> > 	 *					  smp_mb__before_atomic();
> > 	 *				      [L] atomic_read(used_lists)

Note that this is broken; smp_mb__before_atomic() is not valid on
atomic_read().

> > 	 *   [S] atomic_inc(used_lists);
> > 	 *       smp_mb__after_atomic();
> >