linux-kernel - Re: [GIT PULL tools] Linux kernel memory model

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Message-Id: <20180204091601.GO3617@linux.vnet.ibm.com>
Date:   Sun, 4 Feb 2018 01:16:01 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, parri.andrea@...il.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, boqun.feng@...il.com,
        will.deacon@....com, peterz@...radead.org, npiggin@...il.com,
        dhowells@...hat.com, elena.reshetova@...el.com, mhocko@...e.com,
        akiyks@...il.com, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [GIT PULL tools] Linux kernel memory model

On Sat, Feb 03, 2018 at 05:10:06PM -0500, Alan Stern wrote:
> On Sat, 3 Feb 2018, Paul E. McKenney wrote:
> 
> > Please see below for an initial patch to this effect.  This activity
> > proved to be more productive than expected for these tests, which certainly
> > supports our assertion that locking needs more testing...
> > 
> > MP+polocks.litmus
> > MP+porevlocks.litmus
> > 
> > 	These are allowed by the current model, which surprised me a bit,
> > 	given that even powerpc would forbid them.  Is the rationale
> > 	that a lock-savvy compiler could pull accesses into the lock's
> > 	critical section and then reorder those accesses?  Or does this
> > 	constitute a bug in our model of locking?
> > 
> > 	(And these were allowed when I wrote recipes.txt, embarrassingly
> > 	enough...)
> > 
> > Z6.0+pooncelock+poonceLock+pombonce.litmus
> > 
> > 	This was forbidden when I wrote recipes.txt, but now is allowed.
> > 	The header comment for smp_mb__after_spinlock() makes it pretty
> > 	clear that it must be forbidden.  So this one is a bug in our
> > 	model of locking.
> 
> I just tried testing these under the most recent version of herd, and 
> all three were forbidden.

And they do for me as well once I upgraded to the most recent version of
herd.  Whew!!!

Boy, we weren't kidding when we said that you need to us the latest
and greatest herd7, now were we?  ;-)

							Thanx, Paul