Messages in this thread Patch in this message | | | From | "Paul E. McKenney" <> | Subject | [PATCH memory-model 4/7] tools/memory-model: Update README for addition of SRCU | Date | Wed, 13 Feb 2019 09:37:40 -0800 |
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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> Acked-by: Andrea Parri <andrea.parri@amarulasolutions.com> --- 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 -- 2.17.1
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