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 <paul...@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 <paul...@linux.ibm.com>
> >>
> >> Indeed!,
> >>
> >> Acked-by: Andrea Parri <andrea.pa...@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
> >>>
> >>
> >
>
