Re: [PATCH memory-model 7/8] tools/memory-model: Add documentation about SRCU read-side critical sections

Alan Stern
2023-03-22T14:31:03Z

On Wed, Mar 22, 2023 at 02:40:52AM +0100, Andrea Parri wrote:
> On Mon, Mar 20, 2023 at 06:02:45PM -0700, Paul E. McKenney wrote:
> > From: Alan Stern <stern@rowland.harvard.edu>
> >
> > Expand the discussion of SRCU and its read-side critical sections in
> > the Linux Kernel Memory Model documentation file explanation.txt. The
> > new material discusses recent changes to the memory model made in
> > commit 6cd244c87428 ("tools/memory-model: Provide exact SRCU
> > semantics").
>
> How about squashing the diff below (adjusting subject and changelog):
>
> Andrea
>
> diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memory-model/Documentation/litmus-tests.txt
> index 26554b1c5575e..acac527328a1f 100644
> --- a/tools/memory-model/Documentation/litmus-tests.txt
> +++ b/tools/memory-model/Documentation/litmus-tests.txt
> @@ -1028,32 +1028,7 @@ Limitations of the Linux-kernel memory model (LKMM) include:
> additional call_rcu() process to the site of the
> emulated rcu-barrier().
>
> - 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
> - on the trickiness of such overlapping, please see:
> - https://paulmck.livejournal.com/40593.html
> -
> - f. Reader-writer locking is not modeled. It can be
> + e. Reader-writer locking is not modeled. It can be
> emulated in litmus tests using atomic read-modify-write
> operations.

Excellent suggestion!

Acked-by: Alan Stern <stern@rowland.harvard.edu>

Alan