linux-kernel - Re: [PATCH 2/2] [PATCH] sched: Add smp

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Message-ID: <20150218134706.GW5029@twins.programming.kicks-ass.net>
Date:	Wed, 18 Feb 2015 14:47:06 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Kirill Tkhai <ktkhai@...allels.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>, oleg@...hat.com
Subject: Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking
 cycles

On Tue, Feb 17, 2015 at 01:52:31PM -0800, Paul E. McKenney wrote:
> I could do a table per communication style.  For example, message
> passing looks like this (give or take likely errors in the table):
> 
> 	Side CPU	Top CPU
> 	--------	-------
> 	X = 1;		r1 = Y;
> 	<some barrier>	<some barrier>
> 	Y = 1;		r2 = X;
> 
> 	assert(r1 == 0 || r2 == 1);
> 
> 
>       |   mb  |  wmb  |  rmb  |  rbd  |  acq  |  rel  |  ctl  |
>  -----+-------+-------+-------+-------+-------+-------+-------+
>    mb |   Y   |       |   Y   |   y   |   Y   |       |   Y   +
>  -----+-------+-------+-------+-------+-------+-------+-------+
>   wmb |   Y   |       |   Y   |   y   |   Y   |       |   Y   +
>  -----+-------+-------+-------+-------+-------+-------+-------+
>   rmb |       |       |       |       |       |       |       +
>  -----+-------+-------+-------+-------+-------+-------+-------+
>   rbd |       |       |       |       |       |       |       +
>  -----+-------+-------+-------+-------+-------+-------+-------+
>   acq |       |       |       |       |       |       |       +
>  -----+-------+-------+-------+-------+-------+-------+-------+
>   rel |   Y   |       |   Y   |   y   |   Y   |       |   Y   +
>  -----+-------+-------+-------+-------+-------+-------+-------+
>   ctl |       |       |       |       |       |       |       +
>  -----+-------+-------+-------+-------+-------+-------+-------+
> 
> Here "Y" says that the barrier pair works, "y" says that it can
> work but requires an artificial dependency, and " " says that
> it does not work.

I would maybe do s/artificial/additional/, the pointer deref in RCU is
not really artificial, is it?

Also, how many communication styles do you envision to enumerate?
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