linux-kernel - Re: [PATCH 0/3] tools/memory-model: Add SRCU support

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Date:   Wed, 28 Nov 2018 07:34:14 +0900
From:   Akira Yokosawa <akiyks@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Kernel development list <linux-kernel@...r.kernel.org>,
        Akira Yokosawa <akiyks@...il.com>
Subject: Re: [PATCH 0/3] tools/memory-model: Add SRCU support

On 2018/11/27 09:17:46 -0800, Paul E. McKenney wrote:
> On Tue, Nov 27, 2018 at 01:26:42AM +0100, Andrea Parri wrote:
>>> commit 72f61917f12236514a70017d1ebafb9b8d34a9b6
>>> Author: Paul E. McKenney <paulmck@...ux.ibm.com>
>>> Date:   Mon Nov 26 14:26:43 2018 -0800
>>>
>>>     tools/memory-model: Update README for addition of SRCU
>>>     
>>>     This commit updates the section on LKMM limitations to no longer say
>>>     that SRCU is not modeled, but instead describe how LKMM's modeling of
>>>     SRCU departs from the Linux-kernel implementation.
>>>     
>>>     TL;DR:  There is no known valid use case that cares about the Linux
>>>     kernel's ability to have partially overlapping SRCU read-side critical
>>>     sections.
>>>     
>>>     Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
>>
>> Indeed!,
>>
>> Acked-by: Andrea Parri <andrea.parri@...rulasolutions.com>
> 
> Thank you, applied!
> 
> I moved this commit and Alan's three SRCU commits to the branch destined
> for the upcoming merge window.

We need to bump the version of herdtools7 in "REQUIREMENTS". Would it be
7.52?

Removing the explicit version number might be a better idea. Just
say "The latest version of ...".

Thoughts?

        Thanks, Akira
> 
> 							Thanx, Paul
> 
>>   Andrea
>>
>>
>>>
>>> diff --git a/tools/memory-model/README b/tools/memory-model/README
>>> index 0f2c366518c6..9d7d4f23503f 100644
>>> --- a/tools/memory-model/README
>>> +++ b/tools/memory-model/README
>>> @@ -221,8 +221,29 @@ The Linux-kernel memory model has the following limitations:
>>>  		additional call_rcu() process to the site of the
>>>  		emulated rcu-barrier().
>>>  
>>> -	e.	Sleepable RCU (SRCU) is not modeled.  It can be
>>> -		emulated, but perhaps not simply.
>>> +	e.	Although sleepable RCU (SRCU) is now modeled, there
>>> +		are some subtle differences between its semantics and
>>> +		those in the Linux kernel.  For example, the kernel
>>> +		might interpret the following sequence as two partially
>>> +		overlapping SRCU read-side critical sections:
>>> +
>>> +			 1  r1 = srcu_read_lock(&my_srcu);
>>> +			 2  do_something_1();
>>> +			 3  r2 = srcu_read_lock(&my_srcu);
>>> +			 4  do_something_2();
>>> +			 5  srcu_read_unlock(&my_srcu, r1);
>>> +			 6  do_something_3();
>>> +			 7  srcu_read_unlock(&my_srcu, r2);
>>> +
>>> +		In contrast, LKMM will interpret this as a nested pair of
>>> +		SRCU read-side critical sections, with the outer critical
>>> +		section spanning lines 1-7 and the inner critical section
>>> +		spanning lines 3-5.
>>> +
>>> +		This difference would be more of a concern had anyone
>>> +		identified a reasonable use case for partially overlapping
>>> +		SRCU read-side critical sections.  For more information,
>>> +		please see: https://paulmck.livejournal.com/40593.html
>>>  
>>>  	f.	Reader-writer locking is not modeled.  It can be
>>>  		emulated in litmus tests using atomic read-modify-write
>>>
>>
>