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

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Wed, Nov 28, 2018 at 07:34:14AM +0900, Akira Yokosawa wrote:
> 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@linux.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@linux.ibm.com>
> >>
> >> Indeed!,
> >>
> >> Acked-by: Andrea Parri <andrea.parri@amarulasolutions.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?

Good catch!  And I am currently using 7.51+2(dev), so I suspect that
you are right.  But 7.52 appears to still be in the future.

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

That approach would be easier for us, but might be painful for someone
(say) five years from now trying to run the v4.20 kernel's memory model.

							Thanx, Paul

>         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
> >>>
> >>
> > 
>