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Date: Thu, 9 Nov 2017 18:30:27 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Boqun Feng <boqun.feng@gmail.com>, Andreas Dilger <adilger@dilger.ca>,
        Jan Kara <jack@suse.cz>, Waiman Long <longman@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.com>,
        Jeff Layton <jlayton@poochiereds.net>,
        "J. Bruce Fields" <bfields@fieldses.org>, Tejun Heo <tj@kernel.org>,
        Christoph Lameter <cl@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>, Andi Kleen <andi@firstfloor.org>,
        Dave Chinner <dchinner@redhat.com>
Subject: Re: [PATCH v4] lib/dlock-list: Scale dlock_lists_empty()
Message-ID: <20171109173026.o3vjjebm4inrv7uj@hirez.programming.kicks-ass.net>
References: <20171102170431.oq3i5mxtjcg53uot@linux-n805>
 <81bb3365-63f3-fea8-d238-e3880a4c8033@redhat.com>
 <20171103133420.pngmrsfmtimataz4@linux-n805>
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 <d277e58e-03a7-d224-e5f2-8d30fe4357b6@redhat.com>
 <20171106184708.kmwfcchjwjzucuja@linux-n805>
 <20171107115921.GC11391@quack2.suse.cz>
 <C189B9DE-0845-4EC9-9D96-4815EF9E3F8B@dilger.ca>
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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();
> >