Stream: Beginner Questions

Topic: ✔ Proving a mutex


view this post on Zulip suhr (Oct 26 2021 at 07:56):

Inspired by https://martin.kleppmann.com/2019/09/14/strange-loop-isabelle.html and https://www.microsoft.com/en-us/research/publication/tla-proofs/, I made an attempt to prove a Peterson mutex.

It did not go too far: https://gist.github.com/suhr/c13d89be9110b6c3ac8a0ffdfd422735

I'm really impressed how far you can go by just applying sledgehammer everywhere. But you can only go so far doing so.

My question is: how do you approach proving problems like this? For tiny problems proving is easy: you can always apply substitutions and rules directly if you need to. But in the case of bigger problems, definitions are rather large to substitute them into every lemma.

I would also greatly appreciate any help on this particular problem.

view this post on Zulip Katharina Kreuzer (Oct 26 2021 at 08:28):

You could try to find statements that help you on your way to your final proof, eg:

lemma "show_this_statement"
proof (induct t)
case base -- this is you base case of your data type, eg "case 0" for t::nat
    show ?thesis by ...
next
case step --this is your induction step, eg "case (Suc t)" for t::nat
    have "some_more_statements" by ...
    then show ?thesis by ...
qed

You can have as many proof-steps as you like in Isar-style proofs.
Does this help? (I don't know what exactly you want to prove)

view this post on Zulip suhr (Oct 26 2021 at 09:15):

The lemma I actually want to prove is the following

lemma flag_inv:
  assumes sp: "spec m"
  and dom: "i ∈ {0,1}"
  shows "pc (m t) i ∈ {a2, a3a, a3b, cs} ⟶ flag (m t) i"

It is an invariant about all possible behaviors of the specified system. What is basically says is that if a process i is in one of the given states, then the flag for a given process is set.

System behaviors are modeled as functions nat ⇒ state, returning the system state in a given moment of time. Predicate init defines the
state at the moment of time equal to zero, while relation nxt defines how the state m t is related to state m (Suc t).

Lemmas same_flag, flag_a2 and flag_a3a are actually auxiliary lemmas for proving flag_inv. The problem is, I got stuck at flag_a3a.

Proofs generated by sledgehammer for same_flag and flag_a2 invoke an SMT solver, even though the definitions are mostly using just logic and equality. I suspect that my way to define things might make things harder than they should be, but I don't know how to diagnose it.

view this post on Zulip Katharina Kreuzer (Oct 26 2021 at 09:21):

Most of the statements the smt solver and metis use are all your definitions (*_def). Without these definitions, the system cannot know what to reason about. What happens, if you hand in all those definitions by hand?
Either by using ... or you could try what happens if you say something like apply (auto simp add: ...)

view this post on Zulip suhr (Oct 28 2021 at 04:51):

I figured out two lemmas, that seem to be quite useful. But now I have a different problem:

lemma ninv_next:
  assumes sp: "spec m"
  and refl: "∀s. P s s"
  and st: "∀s s'. ∀i ∈ {0,1}. (nall i s s' ⟶ P s s')"
shows "P (m t) (m (Suc t))"
  using sp refl st spec_def nxt_def by metis

lemma same_flag:
  assumes sp: "spec m"
  and dom: "i ∈ {0,1}"
  shows "pc (m t) i ∈ {a1,a4} ∨ flag (m t) i = flag(m (Suc t)) i" (is ?thesis)
proof -
  let "?P" = "λj s s'. pc s j ∈ {a1,a4} ∨ flag s j = flag s' j"
  have refl: "∀s. ?P i s s" by auto
  have st: "∀s s'. ∀j ∈ {0,1}. (nall j s s' ⟶ ?P j s s')"
    using dom nall_def  n0_def n1_def n2_def n3a_def n3b_def n4_def ncs_def
    by auto
  show ?thesis using sp dom refl st ninv_next sorry
qed

Even though ?P i satisfies both refl and st, for some reason I can't derive the conclusion that ?P i (m t) (m (Suc t)).

view this post on Zulip Mathias Fleury (Oct 28 2021 at 05:00):

In st, it should be ?P i instead of ?P j right?

Otherwise, from the distance, that looks like finding the right instantiation for P fails. Try ninv_next[of _ "?P i"] instead of ninv_next. At least that could help you going forward. in finding the issue.

view this post on Zulip suhr (Oct 28 2021 at 05:04):

Yes, this indeed helps. Thanks!

view this post on Zulip suhr (Oct 28 2021 at 06:19):

Is there a way to abbreviate using nall_def n0_def n1_def n2_def n3a_def n3b_def n4_def ncs_def into something like using defs?

view this post on Zulip Mathias Fleury (Oct 28 2021 at 06:27):

lemmas defs = nall_def n0_def n1_def n2_def n3a_def n3b_def n4_def ncs_def

but it is not clear if this is really a good idea

view this post on Zulip Notification Bot (Nov 14 2021 at 13:31):

suhr has marked this topic as resolved.


Last updated: Sep 25 2022 at 23:25 UTC