linux-kernel - Re: [PATCH] Linux: Implement membarrier function

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Date:   Wed, 12 Dec 2018 11:42:25 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     David Goldblatt <davidtgoldblatt@...il.com>,
        mathieu.desnoyers@...icios.com,
        Florian Weimer <fweimer@...hat.com>, triegel@...hat.com,
        libc-alpha@...rceware.org, andrea.parri@...rulasolutions.com,
        will.deacon@....com, peterz@...radead.org, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com, dlustig@...dia.com,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Linux: Implement membarrier function

On Wed, Dec 12, 2018 at 01:04:44PM -0500, Alan Stern wrote:
> On Wed, 12 Dec 2018, Paul E. McKenney wrote:
> 
> > > > > Or am I still missing something here?
> > > > 
> > > > You tell me...
> > > 
> > > I think I am on board.  ;-)
> > 
> > And more to the point, here is a three-process variant showing a cycle
> > that is permitted:
> > 
> > 
> >          P0      P1      P2
> >          Wa=2    Wb=2    Wc=2
> >          mb0s
> >                  [mb01]  [mb02]
> >          mb0e
> >          Rb=0    Rc=0    Ra=0
> > 
> > As can be seen by reordering it as follows:
> > 
> >          P0      P1      P2
> >                          Ra=0
> >          Wa=2
> >          mb0s
> >                  [mb01]
> >                  Rc=0
> >                          Wc=2
> >                          [mb02]
> >          mb0e
> >          Rb=0
> >                  Wb=2
> > 
> > Make sense?
> 
> You got it!

OK.  How about this one?

         P0      P1                 P2      P3
         Wa=2    rcu_read_lock()    Wc=2    Wd=2
         memb    Wb=2               Rd=0    synchronize_rcu();
         Rb=0    Rc=0                       Ra=0
	         rcu_read_unlock()

The model should say that it is allowed.  Taking a look...

         P0      P1                 P2      P3
				    Rd=0
					    Wd=2
					    synchronize_rcu();
	                                    Ra=0
	 Wa=2
	 membs
	         rcu_read_lock()
		 [m01]
		 Rc=0
		 		    Wc=2
				    [m02]   [m03]
	 membe
	 Rb=0
	         Wb=2
		 rcu_read_unlock()

Looks allowed to me.  If the synchronization of P1 and P2 were
interchanged, it should be forbidden:

         P0      P1      P2                 P3
         Wa=2    Wb=2    rcu_read_lock()    Wd=2
         memb    Rc=0    Wc=2               synchronize_rcu();
         Rb=0            Rd=0               Ra=0
                         rcu_read_unlock()

Taking a look...

         P0      P1      P2                 P3
                         rcu_read_lock()
                         Rd=0
         Wa=2    Wb=2                       Wd=2
         membs                              synchronize_rcu();
                 [m01]
                 Rc=0
                         Wc=2
                         rcu_read_unlock()
			 [m02]              Ra=0 [Forbidden?]
	 membe
         Rb=0

I believe that this ordering forbids the cycle:

	Wa=1 > membs -> [m01] -> Rc=0 -> Wc=2 -> rcu_read_unlock() ->
		return from synchronize_rcu() -> Ra

Does this make sense, or am I missing something?

							Thanx, Paul