Make stacked borrow failures easier to debug

[oli-obk]

After a single run of miri fails with a stacked borrow failure, it is pretty hard to debug what's going on. All you get is an id of a borrow and a backtrace pretty late in the process. Since we can be deterministic, maybe we could add a -Ztrace-borrows flag that you can give a list of ids and it will print backtraces or other things whenever an id from the list is affected.

[RalfJung]

Yeah, debugging is quite awful for this. Ideally we'd capture (partial) stacktraces to tell you where the ID became invalid, or so.

[RalfJung]

For those reading along: the Stacked Borrows debugging situation got a lot better with #1080. There's still a lot to be desired, though.

[jrvanwhy]

Note: I'm still trying to comprehend stacked borrows, so my suggestion may not make sense.

One tool that I would find useful is a way to print out the borrow stack for a place. Something like:

#[cfg(miri)]
extern "Rust" {
    fn miri_print_borrow_stack(place: *const u8);
}

miri_print_borrow_stack would print out the information in the Stack type described here.

[RalfJung]

@jrvanwhy I like the basic idea.

However, what do you think the output will look like? It mostly consists of many numbers that are entirely meaningless on their own: stack elements look like [SharedReadOnly for <17539>]. These numbers only make sense when you know which pointers (in which Rust variables) have which tag, and I am not sure how Miri would convey that information.

[bjorn3]

I think it would be useful if priroda could visualize the stacked borrows state. That would make it possible to take a deeper look at the state than a static error message.

[jrvanwhy]

@jrvanwhy I like the basic idea.

However, what do you think the output will look like? It mostly consists of many numbers that are entirely meaningless on their own: stack elements look like [SharedReadOnly for <17539>]. These numbers only make sense when you know which pointers (in which Rust variables) have which tag, and I am not sure how Miri would convey that information.

That looks right to me. Seeing the permissions printed alone will help me identify when I missed a stack entry (e.g. when a I fail to identify that a retag has happened). If execution is deterministic, I could use -Zmiri-track-pointer-tag and -Zmiri-track-call-id to identify the unexpected entries.

It's possible that my suggestion is a better learning tool for those of us trying to understand stacked borrows than it is a debugging tool for those who already understand stacked borrows.

I think it would be useful if priroda could visualize the stacked borrows state. That would make it possible to take a deeper look at the state than a static error message.

Thanks for mentioning priroda, I have not heard of it before!

[RalfJung]

I think it would be useful if priroda could visualize the stacked borrows state. That would make it possible to take a deeper look at the state than a static error message.

Yes, that would be really nice.

That looks right to me. Seeing the permissions printed alone will help me identify when I missed a stack entry (e.g. when a I fail to identify that a retag has happened). I

I must be missing something, since the entries look completely meaningless to me. Could you give a concrete example?

[jrvanwhy]

That looks right to me. Seeing the permissions printed alone will help me identify when I missed a stack entry (e.g. when a I fail to identify that a retag has happened). I

I must be missing something, since the entries look completely meaningless to me. Could you give a concrete example?

I took your example from https://www.ralfj.de/blog/2019/05/21/stacked-borrows-2.1.html and tweaked it:

fn main() { unsafe {
    extern "Rust" {
        fn miri_print_borrow_stack(*const u8);
    }

    let c = &UnsafeCell::new(UnsafeCell::new(0));
    let inner_uniq = &mut *c.get();
    // Prints [{tag: 2723, perm: SharedReadWrite},
    //         {tag: 2731, perm: Unique}]:
    miri_print_borrow_stack(inner_uniq as *const _ as *const u8);

    let inner_shr = &*inner_uniq; // adds a SharedReadWrite
    // Prints [{tag: 2823, perm: SharedReadWrite},
    //         {tag: 2731, perm: Unique},
    //         {tag: 2732, perm: SharedReadWrite}]:
    miri_print_borrow_stack(inner_shr as *const _ as *const u8);

    *c.get() = UnsafeCell::new(1); // invalidates inner_shr
    // Prints [{tag: 2823, perm: SharedReadWrite}]:
    miri_print_borrow_stack(inner_shr as *const _ as *const u8);

    let _ = inner_shr;
} }

If this doesn't make sense, then I probably need to learn more before I can make useful suggestions.