Age is of no importance unless you're a cheese -- Billie Burke

Modeling (lack of) store ordering using PlusCal - and a wishlist

| Comments

The Message Passing pattern (MP pattern) is shown in the snippet below (borrowed from LKMM docs). Here, P0 and P1 are 2 CPUs executing some code. P0 stores a message in buf and then signals to consumers like P1 that the message is available – by doing a store to flag. P1 reads flag and if it is set, knows that some data is available in buf and goes ahead and reads it. However, if flag is not set, then P1 does nothing else. Without memory barriers between P0’s stores and P1’s loads, the stores can appear out of order to P1 (on some systems), thus breaking the pattern. The condition r1 == 0 and r2 == 1 is a failure in the below code and would violate the condition. Only after the flag variable is updated, should P1 be allowed to read the buf (“message”).

        int buf = 0, flag = 0;

                WRITE_ONCE(buf, 1);
                WRITE_ONCE(flag, 1);

                int r1;
                int r2 = 0;

                r1 = READ_ONCE(flag);
                if (r1)
                        r2 = READ_ONCE(buf);

Below is a simple program in PlusCal to model the “Message passing” access pattern and check whether the failure scenario r1 == 0 and r2 == 1 could ever occur. In PlusCal, we can model the non deterministic out-of-order stores to buf and flag using an either or block. This makes PlusCal evaluate both scenarios of stores (store to buf first and then flag, or viceversa) during model checking. The technique used for modeling this non-determinism is similar to how it is done in Promela/Spin using an “if block” (Refer to Paul McKenney’s perfbook for details on that).

(*--algorithm mp_pattern
    buf = 0,
    flag = 0;

process Writer = 1
e1:        buf := 1;
e2:        flag := 1;
e3:        flag := 1;
e4:        buf := 1;
        end either;
end process;

process Reader = 2
    r1 = 0,
    r2 = 0;  
e5:     r1 := flag;
e6:     if r1 = 1 then
e7:         r2 := buf;
        end if;
e8:     assert r1 = 0 \/ r2 = 1;
end process;

end algorithm;*)

Sure enough, the assert r1 = 0 \/ r2 = 1; fires when the PlusCal program is run through the TLC model checker.

I do find the either or block clunky, and wish I could just do something like:

non_deterministic {
        buf := 1;
        flag := 1;

And then, PlusCal should evaluate both store orders. In fact, if I wanted more than 2 stores, then it can get crazy pretty quickly without such a construct. I should try to hack the PlusCal sources soon if I get time, to do exactly this. Thankfully it is open source software.

Other notes:

  • PlusCal is a powerful language that translates to TLA+. TLA+ is to PlusCal what assembler is to C. I do find PlusCal’s syntax to be non-intuitive but that could just be because I am new to it. In particular, I hate having to mark statements with labels if I don’t want them to atomically execute with neighboring statements. In PlusCal, a label is used to mark a statement as an “atomic” entity. A group of statements under a label are all atomic. However, if you don’t specific labels on every statement like I did above (eX), then everything goes under a neighboring label. I wish PlusCal had an option, where a programmer could add implict labels to all statements, and then add explicit atomic { } blocks around statements that were indeed atomic. This is similar to how it is done in Promela/Spin.

  • I might try to hack up my own compiler to TLA+ if I can find the time to, or better yet modify PlusCal itself to do what I want. Thankfully the code for the PlusCal translator is open source software.