linux-kernel - Re: Trial of conflict resolution of Alan's patch

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Message-Id: <20180215192914.GA3617@linux.vnet.ibm.com>
Date:   Thu, 15 Feb 2018 11:29:14 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Akira Yokosawa <akiyks@...il.com>, linux-kernel@...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,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: Trial of conflict resolution of Alan's patch

On Thu, Feb 15, 2018 at 12:51:56PM -0500, Alan Stern wrote:
> On Fri, 16 Feb 2018, Akira Yokosawa wrote:
> 
> > So, I attempted to rebase the patch to current (somewhat old) master of
> > https://github.com/aparri/memory-model. Why? Because the lkmm branch
> > in Paul's -rcu tree doesn't have linux-kernel-hardware.cat.
> > 
> > However, after this change, Z6.0+pooncelock+pooncelock+pombonce still
> > has the result "Sometimes". I must have done something wrong in the
> > conflict resolution.
> > 
> > Note: I have almost no idea what this patch is doing. I'm just hoping
> > to give a starting point of a discussion.
> 
> Yes, that litmus test gives "Sometimes" both with and without the 
> patch.  But consider instead this slightly changed version of that 
> test, in which P2 reads Z instead of writing it:
> 
> C Z6.0-variant
> 
> {}
> 
> P0(int *x, int *y, spinlock_t *mylock)
> {
> 	spin_lock(mylock);
> 	WRITE_ONCE(*x, 1);
> 	WRITE_ONCE(*y, 1);
> 	spin_unlock(mylock);
> }
> 
> P1(int *y, int *z, spinlock_t *mylock)
> {
> 	int r0;
> 
> 	spin_lock(mylock);
> 	r0 = READ_ONCE(*y);
> 	WRITE_ONCE(*z, 1);
> 	spin_unlock(mylock);
> }
> 
> P2(int *x, int *z)
> {
> 	int r1;
> 	int r2;
> 
> 	r2 = READ_ONCE(*z);
> 	smp_mb();
> 	r1 = READ_ONCE(*x);
> }
> 
> exists (1:r0=1 /\ 2:r2=1 /\ 2:r1=0)
> 
> Without the patch, this test gives "Sometimes"; with the patch it gives 
> "Never".  That is what I thought Paul was talking about originally.  
> 
> Sorry if my misunderstanding caused too much confusion for other 
> people.

Ah, I did indeed get confused.  I have changed the "Result:" for
Z6.0+pooncelock+pooncelock+pombonce.litmus back to "Never", as in
the patch below (which I merged into the patch adding all the
comments).

I have added the above test as ISA2+pooncelock+pooncelock+pombonce.litmus,
with the Result: of Sometimes with you (Alan) as author and with your
Signed-off-by -- please let me know if you would prefer some other
approach.

Please change the Result: when sending the proposed patch.  Or please let
me know if you would like me to apply the forward-port that Akira sent,
in which case I will add the Result: change to that patch.  Or for that
matter, Akira might repost his forward-port of your patch with this change.

							Thanx, Paul

------------------------------------------------------------------------

commit b2950420e1154131c0667f1ac58666bad3a06a69
Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Date:   Thu Feb 15 10:35:25 2018 -0800

    fixup! EXP litmus_tests:  Add comments explaining tests' purposes
    
    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/tools/memory-model/litmus-tests/Z6.0+pooncelock+pooncelock+pombonce.litmus b/tools/memory-model/litmus-tests/Z6.0+pooncelock+pooncelock+pombonce.litmus
index fad47258a3e3..95890669859b 100644
--- a/tools/memory-model/litmus-tests/Z6.0+pooncelock+pooncelock+pombonce.litmus
+++ b/tools/memory-model/litmus-tests/Z6.0+pooncelock+pooncelock+pombonce.litmus
@@ -1,7 +1,7 @@
 C Z6.0+pooncelock+pooncelock+pombonce
 
 (*
- * Result: Never
+ * Result: Somtimes
  *
  * This example demonstrates that a pair of accesses made by different
  * processes each while holding a given lock will not necessarily be