Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles

[Oleg Nesterov <oleg@redhat.com>]

On 02/18, Peter Zijlstra wrote:
>
> 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.

Ah, yeeees, sorry. Somehow I completely missed that part of Paul's email.

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

At least we know that I certainly can't ;)

Thanks,

Oleg.