Messages in this thread | ![/](/images/icornerl.gif) | | Date | Tue, 17 Jan 2023 10:56:34 -0500 | From | Alan Stern <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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On Tue, Jan 17, 2023 at 07:14:16AM -0800, Paul E. McKenney wrote: > On Tue, Jan 17, 2023 at 12:46:28PM +0100, Andrea Parri wrote: > > This was reminiscent of old discussions, in fact, we do have: > > > > [tools/memory-model/Documentation/litmus-tests.txt] > > > > 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 > > Good point, if we do change the definition, we also need to update > this documentation. > > > More recently/related, > > > > https://lore.kernel.org/lkml/20220421230848.GA194034@paulmck-ThinkPad-P17-Gen-1/T/#m2a8701c7c377ccb27190a6679e58b0929b0b0ad9 > > It would not be a bad thing for LKMM to be able to show people the > error of their ways when they try non-nested partially overlapping SRCU > read-side critical sections. Or, should they find some valid use case, > to help them prove their point. ;-)
Isn't it true that the current code will flag srcu-bad-nesting if a litmus test has non-nested overlapping SRCU read-side critical sections?
And if it is true, is there any need to change the memory model at this point?
(And if it's not true, that's most likely due to a bug in herd7.)
Alan
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