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

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Message-ID: <20150218161112.GB5029@twins.programming.kicks-ass.net>
Date:	Wed, 18 Feb 2015 17:11:12 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Kirill Tkhai <ktkhai@...allels.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking
 cycles

On Wed, Feb 18, 2015 at 04:53:34PM +0100, Oleg Nesterov wrote:
> On 02/17, Paul E. McKenney wrote:
> >
> >       |   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 |       |       |       |       |       |       |       +
> >  -----+-------+-------+-------+-------+-------+-------+-------+
> 
> OK, so "acq" can't pair with "acq", and I am not sure I understand.

Please consider the table in the context of message passing; that is
what Paul proposed. Your example from sysvsems, while interesting, would
not fit the general scenario of message passing.

This too illustrates a problem with that approach, people can't read, so
they'll pick the wrong table to look at.

I really think having just the _one_ table with obvious pairings marked,
and everything not marked in the table needs careful reading.

And acq-acq pairing would be a careful one in my book.
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