Commit ad9fd20b authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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: default avatarPaul E. McKenney <paulmck@linux.ibm.com>
Acked-by: default avatarAndrea Parri <andrea.parri@amarulasolutions.com>
parent a3f600d9

tools/memory-model/README

+23 −2
Original line number Diff line number Diff line
@@ -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