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

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Message-ID: <fe361fba-d6ec-3a70-6ab9-e148bccb5833@gmail.com>
Date:   Fri, 16 Feb 2018 06:51:11 +0900
From:   Akira Yokosawa <akiyks@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Alan Stern <stern@...land.harvard.edu>
Cc:     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>,
        Akira Yokosawa <akiyks@...il.com>
Subject: Re: Trial of conflict resolution of Alan's patch

On 2018/02/15 11:29:14 -0800, Paul E. McKenney wrote:
> 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.
> 

Hi,

My forward-port patch doesn't apply to the "lkmm" branch.
It looks like "linux-kernel-hardware.cat" is intentionally omitted there.
Am I guessing right?

If this is the case, I can prepare a patch to be applied to "lkmm".
But I can't compose a proper change log. So I'd like Alan to post
a patch with my SOB appended. Does this approach sound reasonable?

      Thanks, Akira

> 							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
>