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Date: Thu, 9 Nov 2017 09:24:08 -0800
From: Davidlohr Bueso <dave@stgolabs.net>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Andreas Dilger <adilger@dilger.ca>, Jan Kara <jack@suse.cz>,
        Waiman Long <longman@redhat.com>,
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        Andi Kleen <andi@firstfloor.org>, Dave Chinner <dchinner@redhat.com>
Subject: Re: [PATCH v4] lib/dlock-list: Scale dlock_lists_empty()
Message-ID: <20171109172408.q5oo7u5skc5vjswb@linux-n805>
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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)
> 	 *   [S] atomic_inc(used_lists);
> 	 *       smp_mb__after_atomic();
>
>, in which case dlock_lists_empty() misses a increment of used_lists.
>
>That said, this may be fine for the usage of dlock_list. But I think the
>comments are confusing or wrong. The reason is you can not use barriers
>to order two memory ops on different CPUs, and usually we add comments
>like:

So I think that case is OK as CPU1 legitimately reads a 0, the problem
would be if we miss the inc it because we are doing spin_lock().

>
>	[S] ...			[S] ...
>	<barrier>		<barrier>
>	[L] ...			[L] ...

That is true.

>
>Davidlohr, could you try to improve the comments by finding both memory
>ops preceding and following the barriers? Maybe you will find those
>barriers are not necessary in the end.

Ok so for the dlock_list_add() the first barrier is so that the atomic_inc()
is not done inside the critical region, after the head->lock is taken.
The other barriers that follow I put such that the respective atomic op
is done before the list_add(), however thinking about it I don't think
they are really needed.

Regarding dlock_list_empty(), the smp_mb__before_atomic() is mainly for
pairing reasons; but at this point I don't see a respective counterpart
for the above diagram so I'm unclear.

Thanks,
Davidlohr
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