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

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
> >
>

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