Commit-ID: ad9fd20b6dadb0cb14551477fcebe0fdf2e697dd Gitweb: https://git.kernel.org/tip/ad9fd20b6dadb0cb14551477fcebe0fdf2e697dd Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxx> AuthorDate: Mon, 26 Nov 2018 14:26:43 -0800 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxx> CommitDate: Mon, 18 Mar 2019 10:27:52 -0700 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@xxxxxxxxxxxxx> Acked-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx> --- tools/memory-model/README | 25 +++++++++++++++++++++++-- 1 file changed, 23 insertions(+), 2 deletions(-) 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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