Messages in this thread | | | Date | Tue, 17 Jan 2023 09:43:08 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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On Tue, Jan 17, 2023 at 10:56:34AM -0500, Alan Stern wrote: > 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?
Now that you mention it, it does indeed, flagging srcu-bad-nesting.
Just to see if I understand, different-values yields true if the set contains multiple elements with the same value mapping to different values. Or, to put it another way, if the relation does not correspond to a function.
Or am I still missing something?
> 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.)
Agreed, changes must wait for SRCU support in herd7.
At which point something roughly similar to this might work?
let srcu-rscs = return_value(Srcu-lock) ; (dep | rfi)* ; parameter(Srcu-unlock, 2)
Given an Srcu-down and an Srcu-up:
let srcu-rscs = ( return_value(Srcu-lock) ; (dep | rfi)* ; parameter(Srcu-unlock, 2) ) | ( return_value(Srcu-down) ; (dep | rf)* ; parameter(Srcu-up, 2) )
Seem reasonable, or am I missing yet something else?
Thanx, Paul
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