Messages in this thread | | | Date | Tue, 17 Jan 2023 10:55:21 -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 07:27:29PM +0100, Jonas Oberhauser wrote: > On 1/17/2023 6:43 PM, Paul E. McKenney wrote: > > 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? > > based on https://lkml.org/lkml/2019/1/10/155:
Ah, thank you for the pointer!
> I think different-values(r) is the same as r \ same-values, where > same-values links all reads and writes that have the same value (e.g., > "write 5 to x" and "read 5 from y"). > > With this in mind, I think the idea is to 1) forbid partial overlap, and > using the different-values to 2) force them to provide the appropriate > value. > This works because apparently srcu-lock is a read and srcu-unlock is a > write, so in case of > int r1 = srcu-lock(&ss); ==> Read(&ss, x), r1 := x > ... > srcu-unlock(&ss, r1); ==> Write(&ss, r1), which is Write(&ss, x) > > This guarantees that the read and write have the same value, hence > different-values(...) will be the empty relation, and so no flag.
Might it instead match the entire event?
> > > 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) > > I would like instead to be able to give names to the arguments of events > that become dependency relations, like > event srcu_unlock(struct srcu_struct *srcu_addr, struct srcu_token > *srcu_data) > and then > let srcu-rscs = [Srcu-lock] ; srcu_data ; (data; rfi)* > > Personally I would also like to not have Linux-specific primitives in > herd7/cat, that means that to understand LKMM you also need to understand > the herd7 tool, and sounds quite brittle. > > I would prefer if herd7 had some means to define custom events/instructions > and uninterpreted relations between them, like > > relation rf : [write] x [read] > [read] <= range(rf) > empty rf ;rf^-1 \ id > > and some way to say > [read] ; .return <= rf^-1 ; .data > (where .return is a functional relation relating every event to the value it > returns, and .xyz is the functional relation relating every event to the > value of its argument xyz).
I am glad that I asked rather than kneejerk filing a bug report. ;-)
Other thoughts?
Thanx, Paul
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