linux-kernel - Re: [PATCH RFC memory-model 0/6] LKMM updates

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Date:   Thu, 10 Jan 2019 08:31:26 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org, parri.andrea@...il.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, willy@...radead.org
Subject: Re: [PATCH RFC memory-model 0/6] LKMM updates

On Thu, Jan 10, 2019 at 10:41:23AM -0500, Alan Stern wrote:
> On Thu, 10 Jan 2019, Paul E. McKenney wrote:
> 
> > On Thu, Jan 10, 2019 at 09:40:24AM +0100, Andrea Parri wrote:
> > > > > > > It seems that
> > > > > > > 
> > > > > > >   1b52d0186177 ("tools/memory-model: Model smp_mb__after_unlock_lock()")
> > > > > > >   
> > > > > > > from linux-rcu/dev got lost; this also needs an ack (probably yours! ;D,
> > > > > > > considered that, IIRC, you introduced the primitive and RCU is currently
> > > > > > > its only user.)
> > > > > > 
> > > > > > That commit is in -tip:
> > > > > > 
> > > > > > 4607abbcf464 ("tools/memory-model: Model smp_mb__after_unlock_lock()")
> > > > > > 
> > > > > > So it has already left my -rcu tree.  ;-)
> > > > > 
> > > > > Oh, you're right: now I see the commit (e.g., with "git show"), but I
> > > > > don't see the corresponding changes applied to the tree.
> > > > > 
> > > > >   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=locking/core&id=4607abbcf464ea2be14da444215d05c73025cf6e
> > > > >   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/tree/tools/memory-model/linux-kernel.bell?h=locking/core
> > > > > 
> > > > > Is this expected?
> > > > 
> > > > Are you asking why it is in -tip but not in mainline?  I am not sure,
> > > > but given that the merge window was over the holiday season and that
> > > > the length of the merge window proved to be shorter than many people
> > > > expected it to be, I am not too surprised.  ;-)
> > > 
> > > Mmh, let me try again:
> > > 
> > >   $ git clone git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git
> > >   $ cd tip
> > >   $ git checkout -b locking/core origin/locking/core
> > > 
> > >   $ git show 4607abbcf464
> > >   commit 4607abbcf464ea2be14da444215d05c73025cf6e
> > >   Author: Andrea Parri <andrea.parri@...rulasolutions.com>
> > >   Date:   Mon Dec 3 15:04:49 2018 -0800
> > > 
> > >       tools/memory-model: Model smp_mb__after_unlock_lock()
> > > 
> > >   $ cd tools/memory-model
> > >   $ herd7 -conf linux-kernel.cfg after-unlock-lock-same-cpu.litmus
> > >   File "after-unlock-lock-same-cpu.litmus": Unknown macro smp_mb__after_unlock_lock (User error)
> > > 
> > >   [aka, linux-kernel.def in tip:locking/core does not have the
> > >    smp_mb__after_unlock_lock() added by 4607abbcf464]
> > 
> > Color me confused:
> > 
> > ------------------------------------------------------------------------
> > 
> > $ git checkout 4607abbcf464Checking out files: 100% (18397/18397), done.
> > Previous HEAD position was 4e284b1bf15a rcu: Remove wrapper definitions for obsolete RCU update functions
> > HEAD is now at 4607abbcf464 tools/memory-model: Model smp_mb__after_unlock_lock()
> > $ grep smp_mb__after_unlock_lock tools/memory-model/linux-kernel.def 
> > smp_mb__after_unlock_lock() { __fence{after-unlock-lock}; }
> > 
> > ------------------------------------------------------------------------
> > 
> > In addition, it handles this litmus test just fine:
> > 
> > ------------------------------------------------------------------------
> > 
> > C MP+polocks
> > 
> > (*
> >  * Result: Never
> >  *
> >  * This litmus test demonstrates how lock acquisitions and releases can
> >  * stand in for smp_load_acquire() and smp_store_release(), respectively.
> >  * In other words, when holding a given lock (or indeed after releasing a
> >  * given lock), a CPU is not only guaranteed to see the accesses that other
> >  * CPUs made while previously holding that lock, it is also guaranteed
> >  * to see all prior accesses by those other CPUs.
> >  *)
> > 
> > {}
> > 
> > P0(int *x, int *y, spinlock_t *mylock)
> > {
> > 	WRITE_ONCE(*x, 1);
> > 	spin_lock(mylock);
> > 	smp_mb__after_unlock_lock();
> > 	WRITE_ONCE(*y, 1);
> > 	spin_unlock(mylock);
> > }
> > 
> > P1(int *x, int *y, spinlock_t *mylock)
> > {
> > 	int r0;
> > 	int r1;
> > 
> > 	spin_lock(mylock);
> > 	r0 = READ_ONCE(*y);
> > 	spin_unlock(mylock);
> > 	r1 = READ_ONCE(*x);
> > }
> > 
> > exists (1:r0=1 /\ 1:r1=0)
> > 
> > ------------------------------------------------------------------------
> > 
> > Again, color me confused.
> 
> Andrea's point is that while the 1b52d0186177 commit is present in the
> tip repository, it isn't in the locking/core branch.

That commit is still in tip/master, so it has not been lost or
forgotten.  ;-)

							Thanx, Paul