Re: Can the Kernel Concurrency Sanitizer Own Rust Code?

From: "Paul E. McKenney" <paulmck@kernel.org>
Date: 2021-10-15 23:29:05

On Fri, Oct 15, 2021 at 04:06:40PM +0100, Wedson Almeida Filho wrote:

On Thu, Oct 14, 2021 at 12:43:41PM -0700, Paul E. McKenney wrote:

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On Thu, Oct 14, 2021 at 09:03:42AM +0100, Wedson Almeida Filho wrote:

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On Wed, Oct 13, 2021 at 08:35:57PM -0700, Paul E. McKenney wrote:

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On Wed, Oct 13, 2021 at 06:50:24PM +0100, Wedson Almeida Filho wrote:

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On Wed, Oct 13, 2021 at 09:07:07AM -0700, Paul E. McKenney wrote:

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On Wed, Oct 13, 2021 at 01:48:13PM +0200, Miguel Ojeda wrote:

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On Mon, Oct 11, 2021 at 9:01 PM Paul E. McKenney [off-list ref] wrote:

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The main issue I was calling out was not justifying Rust, but rather
making sure that the exact same build could be reproduced a decade later.

Yes, but that is quite trivial compared to other issues I was
mentioning like adapting and requalifying a testing tool. For
instance, if you already had a team maintaining the configuration
management (i.e. the versions etc.), adding one more tool is not a big
deal.

OK, close enough to fair enough.  ;-)

quoted

There are things that concurrent software would like to do that are
made quite inconvenient due to large numbers of existing optimizations
in the various compiler backends.  Yes, we have workarounds.  But I
do not see how Rust is going to help with these inconveniences.

Sure, but C UB is unrelated to Rust UB. Thus, if you think it would be
valuable to be able to express particular algorithms in unsafe Rust,
then I would contact the Rust teams to let them know your needs --
perhaps we end up with something way better than C for that use case!

Sequence locks and RCU do seem to be posing some challenges.  I suppose
this should not be too much of a surprise, given that there are people who
have been in the Rust community for a long time who do understand both.
If it were easy, they would have already come up with a solution.

(Hey Paul, I tried posting on your blog series, but I'm having difficulty so I
thought I'd reply here given that we mention seqlocks and RCU here.)

It should be straightforward to post a comment, but some report that
their employers block livejournal.com.  :-/

I tried to use my google account while posting and then after I posted it took
me through some workflow to confirm my account, perhaps the comment was lost
during this workflow. Let me try again.

Please let me know how it goes.

It says my comment is spam :) When I'm logged in I can actually see it as if it
was accepted, but when I open the very same page while logged out, I don't see
any comments.

Here's the URL for the entry where I've left a comment:
https://paulmck.livejournal.com/62835.html

Apologies for the inconvenience!  I have unspammed this comment.  On the
other hand, livejournal's hyperactive spam marking does seem to keep
the spam down.

quoted

Oh, and I have updated heavily recently, including adding a bunch of
Linux-kernel use cases for both sequence locking and RCU.

I'll check it out, thanks!

quoted

I spent a bit of time thinking about sequence locks and I think I have something
that is workable. (I remind you that we use the C implementation for the
synchronisation primitives). Suppose we had some struct like so:

struct X {
    a: AtomicU32,
    b: AtomicU32,
}

And suppose we have it protected by a sequence lock. If we wanted to return the
sum of the two fields, the code would look like this:

    let v = y.access(|x| {
        let a = x.a.load(Ordering::Relaxed);
	let b = x.b.load(Ordering::Relaxed);
	a + b
    });

It would be expanded to the following machine code in aarch64 (when LTO is
enabled):

  403fd4:       14000002        b       403fdc
  403fd8:       d503203f        yield
  403fdc:       b9400808        ldr     w8, [x0, #8]
  403fe0:       3707ffc8        tbnz    w8, #0, 403fd8
  403fe4:       d50339bf        dmb     ishld
  403fe8:       b9400c09        ldr     w9, [x0, #12]
  403fec:       b940100a        ldr     w10, [x0, #16]
  403ff0:       d50339bf        dmb     ishld
  403ff4:       b940080b        ldr     w11, [x0, #8]
  403ff8:       6b08017f        cmp     w11, w8
  403ffc:       54ffff01        b.ne    403fdc
  404000:       0b090148        add     w8, w10, w9

It is as efficient as the C version, though not as ergonomic. The
.load(Ordering::Relaxed) can of course be improved to something shorter like
.load_relaxed() or even new atomic types  with .load() being relaxed and
.load_ordered(Ordering) for other ordering.

Nice!

Is this a native Rust sequence-lock implementation or a wrapper around
the C-language Linux-kernel implementation?

It's a wrapper around the C-language Linux kernel implementation. (To get the
generated code with LTO inlining, I compiled the code in userspace because
LTO with cross-language inlining isn't enabled/working in the kernel yet).

Good on the wrapper, and agreed, I also tend to prototype in userspace.

quoted

I also have guard- and iterator-based methods for the read path that would look
like this (these can all co-exist if we so choose):

    let v = loop {
        let guard = y.read();
        let a = guard.a.load(Ordering::Relaxed);
        let b = guard.b.load(Ordering::Relaxed);
        if !guard.need_retry() {
            break a + b;
        }
    };

and

    let mut v = 0;
    for x in y {
        let a = x.a.load(Ordering::Relaxed);
	let b = x.b.load(Ordering::Relaxed);
	v = a + b;
    }

The former generates the exact same machine code as above though the latter
generates slightly worse code (it has instructions sequences like "mov w10,
#0x1; tbnz w10, #0, 403ffc" and , "mov w10, wzr; tbnz w10, #0, 403ffc", which
could be optimised but for some reason isn't).

The C++ bindings for RCU provide a similar guard approach, leveraging
C++ BasicLock.  Explicit lock and unlock can be obtained using
move-assignments.

I haven't seen these bindings, perhaps I should :) But one relevant point about
guards is that Rust has an affine type system that allows it to catch misuse of
guards at compile time. For example, if one wants to explicitly unlock, the
unlock method 'consumes' (move-assigns) the guard, rendering it unusable:
attempting to use such a guard is a compile-time error (even if it's in scope).
In C++, this wouldn't be caught at compile time as moved variables remain
accessible while in scope.

OK, but there are cases where seqlock entry/exit is buried in helper
functions, for example in follow_dotdot_rcu() function in fs/namei.c.
(See recent changes to https://paulmck.livejournal.com/63957.html.)
This sort of thing is often necessary to support iterators.

So how is that use case handled?

Note that even the C code needs to carry some state between these functions, in
particular the seqp. Rust would be no different, but it would carry the guard
(which would boil down to a single 32-bit value as well); so we would have
something like:

fn follow_dotdot_rcu([args]) -> (Dentry, SeqLockReadGuard);
fn into_dot([args], read_guard: SeqLockReadGuard);

That is, follow_dotdot_rcu, creates a guard and returns it, so the lock
continues acquired (in the case of seqcounters it's just conceptually) as the
function returns, and it can be passed to another function, so an example of
calling the functions above would be:

  let (dentry, guard) = follow_dotdot_rcu([args]);
  into_dot([args], guard);

And into_dot can use the guard as if it had created it itself, and it will be
unlocked once into_dot finishes (or later if into_dot moves it elsewhere).

OK, similar to the way guards are used in C++, then.  Whew!  ;-)

quoted

Plus we could easily get an RAII-like effect in C code for RCU as follows:

	#define rcu_read_lock_scoped rcu_read_lock(); {
	#define rcu_read_unlock_scoped } rcu_read_unlock();

	rcu_read_lock_scoped();
		struct foo *p = rcu_dereference(global_p);

		do_some_rcu_stuff_with(p);
	rcu_read_unlock_scoped();

I think using the __cleanup__ attribute is more promising than the above. The
indentation without explicit braces doesn't seem very ergonomic, perhaps we
could leave the braces out of the macros to improve this... But anyway, if
there's a `return` statement within the block, you end up leaving the function
without unlocking.

quoted

But we don't.  One reason is that we often need to do things like
this:

	rcu_read_lock();
	p = rcu_dereference(global_p);
	if (ask_rcu_question(p)) {
		do_some_other_rcu_thing(p);
		rcu_read_unlock();
		do_something_that_sleeps();
	} else {
		do_yet_some_other_rcu_thing(p);
		rcu_read_unlock();
		do_something_else_that_sleeps();
	}

Sure, you could write that like this:

	bool q;

	rcu_read_lock_scoped();
	struct foo *p = rcu_dereference(global_p);
		q = ask_rcu_question(p);
		if (q)
			do_some_other_rcu_thing(p);
		else
			do_yet_some_other_rcu_thing(p);
	rcu_read_unlock_scoped();
	if (q)
		do_something_that_sleeps();
	else
		do_something_else_that_sleeps();

And I know any number of C++ guys who would sing the benefits of the
latter over the former, but I personally think they are drunk on RAII
Koolaid.  As would any number of people in the Linux kernel community. ;-)

It turns out that there are about 3400 uses of rcu_read_lock() and
about 4200 uses of rcu_read_unlock().  So this sort of thing is common.
Yes, it is possible that use of RAII would get rid of some of them,
but definitely not all of them.

Plus there are situations where an iterator momentarily drops out of
an RCU read-side critical section in order to keep from impeding RCU
grace periods.  These tend to be buried deep down the function-call stack.

Don't get me wrong, RAII has its benefits.  But also its drawbacks.

Agreed. Rust allows RAII, but it's by no means required. Your first example can
be translated to Rust as follows:

 	rcu_guard = rcu::read_lock();
 	p = global_p.rcu_dereference(&rcu_guard);
 	if (ask_rcu_question(p)) {
 		do_some_other_rcu_thing(p);
 		rcu_guard.unlock();
 		do_something_that_sleeps();
 	} else {
 		do_yet_some_other_rcu_thing(p);
 		rcu_guard.unlock();
 		do_something_else_that_sleeps();
 	}

This is not very different from C version but has the following extra
advantages:
  1. global_p can only be dereferenced while in an rcu critical section.
  2. p becomes inaccessible after rcu_guard.unlock() is called.
  3. If we fail to call rcu_guard.unlock() in some code path, it will be
     automatically called when rcu_guard goes out of scope. (But only if we
     forget, otherwise it won't because rcu_guard.unlock consumes the guard.)

OK, so this presumably allows overlapping an RCU reader with a lock:

	rcu_read_lock();
	p = rcu_dereference(global_p);
	if (ask_rcu_question(p)) {
		do_some_other_rcu_thing(p);
		spin_lock(&p->lock);
		rcu_read_unlock();
		do_something_that_sleeps();
		spin_unlock(&p->lock);
	} else {
		do_yet_some_other_rcu_thing(p);
		rcu_read_unlock();
		do_something_else_that_sleeps();
	}

Or do we need to "launder" p somehow to make this work?  There is a macro
that documents similar transitions in the Linux kernel:

	rcu_read_lock();
	p = rcu_dereference(global_p);
	if (ask_rcu_question(p)) {
		do_some_other_rcu_thing(p);
		spin_lock(&p->lock);
		q = rcu_pointer_handoff(p);
		rcu_read_unlock();
		do_something_that_sleeps();
		spin_unlock(&q->lock);
	} else {
		do_yet_some_other_rcu_thing(p);
		rcu_read_unlock();
		do_something_else_that_sleeps();
	}

Another odd twist is where objects are inserted into a given data
structure but never removed.  In that case, you need rcu_dereference(),
but you do not need rcu_read_lock() and rcu_read_unlock().  One approach
within the Linux kernel is rcu_dereference_protected(global_p, 1) or
equivalently rcu_dereference_raw(p).  Thoughts?

quoted

Anyway, on to the write path. We need another primitive to ensure that only one
writer at a time attempts to acquire the sequence lock in write mode. We do this
by taking a guard for this other lock, for example, suppose we want to increment
each of the fields:

    let other_guard = other_lock.lock();
    let guard = y.write(&other_guard);

The first acquires the lock in an RAII (scoped) fashion and the second
enters the sequence-lock write-side critical section, correct?

Yes, exactly.

But wouldn't it be more ergonomic and thus less error-prone to be able
to combine those into a single statement?

Definitely. The above example is similar to the usage of a seqcounter in C --
with the added requirement that users need to provide evidence that they're in
fact using a lock (which is something that C doesn't do, so it's more error
prone).

Combining a lock and a seqcounter into one thing (seqlocks) is better when
that's what users need. I'll improve the wrappers to allow both.

Very good!

quoted

Additionally, the ownership rules guarantee that the outer lock cannot be
unlocked while in the sequence-lock write-side critical section (because the
inner guard borrows the outer one, so it can be only be consumed after this
borrow goes away). An attempt to do so would result in a compile-time error.

OK, let's talk about the Rusty Scale of easy of use...

This was introduced by Rusty Russell in his 2003 Ottawa Linux Symposium
keynote: https://ozlabs.org/~rusty/ols-2003-keynote/ols-keynote-2003.html.
The relevant portion is in slides 39-57.

An API that doesn't let you get it wrong (combined lock/count acquisition)
is better than one where the compiler complains if you get it wrong.  ;-)

You're right.

But see the distinction I made above: seqcounter vs seqlock. In cases when a
seqlock isn't suitable but a seqcounter is, C will let you misuse the write-side
critical section, Rust won't :)

OK, but please understand that "won't" is a very strong word.  ;-)

quoted

    guard.a.store(guard.a.load(Ordering::Relaxed) + 1, Ordering::Relaxed);
    guard.b.store(guard.b.load(Ordering::Relaxed) + 1, Ordering::Relaxed);

The part the relates to the sequence lock is compiled to the following:

  404058:       f9400009        ldr     x9, [x0]
  40405c:       eb08013f        cmp     x9, x8
  404060:       54000281        b.ne    4040b0

  404064:       b9400808        ldr     w8, [x0, #8]
  404068:       11000508        add     w8, w8, #0x1
  40406c:       b9000808        str     w8, [x0, #8]
  404070:       d5033abf        dmb     ishst
  404074:       b9400c08        ldr     w8, [x0, #12]
  404078:       11000508        add     w8, w8, #0x1
  40407c:       b9000c08        str     w8, [x0, #12]
  404080:       b9401008        ldr     w8, [x0, #16]
  404084:       11000508        add     w8, w8, #0x1
  404088:       b9001008        str     w8, [x0, #16]
  40408c:       d5033abf        dmb     ishst
  404090:       b9400808        ldr     w8, [x0, #8]
  404094:       11000508        add     w8, w8, #0x1
  404098:       b9000808        str     w8, [x0, #8]

If we ignore the first three instructions momentarily, the rest is as efficient
as C. The reason we need the first three instructions is to ensure that guard
that was passed into the `write` function is a guard to the correct lock. The
lock type already eliminates the vast majority of issues, but a developer could
accidentally lock the wrong lock and use it in the sequence lock, which would be
problematic. So we need this check in Rust that we don't need in C (although the
same mistake could happen in C).

We can provide an 'unsafe' version that doesn't perform this check, then the
onus is on the callers to convince themselves that they have acquired the
correct lock (and they'd be required to use an unsafe block). Then the
performance would be the same as the C version.

The Linux-kernel C-language sequence counter (as opposed to the various
flavors of sequence lock) assume that the caller has provided any needed
mutual exclusion.

Yes, this actually uses sequence counters.

I suppose if we embed the locks ourselves like sequence locks do, we can wrap
such 'unsafe' blocks as part of the implementation and only expose safe
interfaces as efficient as C.

Do you happen to know the usage ratio between sequence counters vs sequence
locks (all flavours combined)? If the latter are used in the vast majority of
cases, I think it makes sense to do something similar in Rust.

Let's count the initializations:

o	Sequence counters:

	 8	SEQCNT_ZERO
	15	seqcount_init

	23	Total

o	Sequence locks:

	3	SEQCNT_RAW_SPINLOCK_ZERO
	3	SEQCNT_SPINLOCK_ZERO
	0	SEQCNT_RWLOCK_ZERO
	0	SEQCNT_MUTEX_ZERO
	0	SEQCNT_WW_MUTEX_ZERO
	1	seqcount_raw_spinlock_init
	13	seqcount_spinlock_init
	1	seqcount_rwlock_init
	1	seqcount_mutex_init
	1	seqcount_ww_mutex_init

	23	Total

Exactly even!  When does -that- ever happen?  ;-)

Oh, man! I was hoping seqlocks would be so dominant that we could ignore
seqcounters in Rust :)

Actually, I bet that you can ignore both seqlocks and seqcounters for
quite some time, depending on what device drivers you are targeting.
Most of the uses are in the core kernel rather than in device drivers.

quoted

Now that I've presented how my proposal looks like from the PoV of a user,
here's its rationale: given that we only want one copy of the data and that
mutable references are always unique in the safe fragment of Rust, we can't (and
don't) return a mutable reference to what's protected by the sequence lock, we
always only allow shared access, even when the sequence lock is acquired in
write mode.

Then how does one change the fields? Interior mutability. In the examples above,
the fields are all atomic, so they can be changed with the `store` method. Any
type that provides interior mutability is suitable here.

OK, so following the approach of "marked accesses".

Yes.

quoted

If we need to use types with interior mutability, what's the point of the
sequence lock? The point is to allow a consistent view of the fields. In our
example, even though `a` and `b` are atomic, the sequence lock guarantees that
readers will get a consistent view of the values even though writers modify one
at a time.

Yes.

I suppose that the KCSAN ASSERT_EXCLUSIVE_WRITER() could be used on
the sequence-lock update side to check for unwanted concurrency.

Yes, definitely!

Could anything be done to check for values leaking out of failed seqlock
read-side critical sections?

I can't think of a way to prevent them outright, but if one uses the
closure-based version, values cannot escape through the captured state of the
closure because it is declared immutable (Fn vs FnMut), though values of failed
iterations could potentially escape through say global variables.

I'll think some more about this to see if I can come up with something. If you
have other ideas, please let us know!

My ignorance of Rust prevents me from saying much.  Me, I am just taking
you guys at your word about preventing bugs.  ;-)

quoted

Lastly, the fact we use a generic `Guard` as proof that a lock is held (for the
write path) means that we don't need to manually implement this for each
different lock we care about; any that implements the `Lock` trait can be used.
This is unlike the C code that uses fragile macros to generate code for
different types of locks (though the scenario is slightly different in that the
C code embeds a lock, which is also something we could do in Rust) -- the Rust
version uses generics, so it is type-checked by the compiler.

OK, so this is a standalone implementation of sequence locks in Rust,
rather than something that could interoperate with the C-language
sequence locks?

It's an implementation of sequence locks using C-language sequence counters.
Instead of embedding a lock for writer mutual exclusion, we require evidence
that some lock is in use. The idea was to be "flexible" and share locks, but if
most usage just embeds a lock, we may as well do something similar in Rust.

Whew!

I don't know if such a case exists, but there is the possibility of
non-lock mutual exclusion.  For example, the last guy to remove a
reference to something is allowed to do a sequence-counter update.

How would such a case be handled?

Well, it depends on how this mutual exclusion can be expressed to Rust. If,
let's say, the protected data structure is being freed, then there it is
guaranteed that no-one else has references to it. In that case, one could just
implement the `Drop` trait and get a mutable reference (&mut) to the object
directly without having to go through the lock.

If Rust can't reason be convinced of the mutual exclusion, then it would require
an unsafe variant, so its declaration would be something like:

/// Enter the write-side critical section.
//
//  # Safety
//
//  Callers must ensure that at all times, at most one thread/CPU call this
//  function and own the guard.
unsafe fn write_unsafe(&self) -> Guard;


And callers would write something like:

  // SAFETY: The mutual exclusion requirements is satisfied by [reason here].
  let guard = unsafe { seqcounter.write_unsafe() };

Note that the `unsafe` annotation in the function declaration above makes it
such that all callers must wrap the calls in `unsafe` blocks. Failure to do so
results in a compiler error saying that they should check the documentation on
the safety requirements for this function.

Perhaps one approach that might work in at least a few cases would be to
bury the reference removal (atomic_dec_and_test()) into the same place
doing the write-side sequence-count work.  Perhaps that would allow the
reference-removal return value to feed in somehow?

But each special case seems like it would need special invention, which
leads to using Rust unsafe (as you suggest above) or just leaving it in
C code.

quoted

Is "fragile macros" just the usual Rust denigration of the C preprocessor,
or is there some specific vulnerability that you see in those macros?

I don't see any specific vulnerability. By fragile I meant that it's more error
prone to write "generic" code with macros than with compiler-supported generics.

Fair enough, but rest assured that those who love the C preprocessor
have their own "interesting" descriptions of Rust macros.  ;-)

Oh, you won't see me defending macros from either language :)

They are crufty, difficult to get right, easy to inject bugs into,
ugly, inelegant, ...

And always there when you need them!

quoted

Plus I am old enough to remember people extolling the simplicity of
C-preprocessor macros compared to, among other things, LISP macros.
And they were correct to do so, at least for simple use cases.

I suggest just calling them CPP macros or similar when talking with
Linux-kernel community members.  Me, I have seen enough software artifacts
come and go that I don't much care what you call them, but others just
might be a bit more touchy about such things.

Sure, but to be clear, I haven't talked about Rust macros, and I don't encourage
their use. I was talking about generics, which is a Rust language feature that
is part of the type system (integral to the lifetimes story), so they are
checked by the compiler, unlike macros (C or Rust).

I agree that various sorts of generics can do some jobs better than
macros can.  Give or take their effect of C++ build times, but maybe
Rust has a better story there.

quoted

Of course, those macros could be used to automatically generate the
wrappers.  Extract the macro invocations from the C source, and transform
them to wrappers, perhaps using Rust macros somewhere along the way.

Sure, we could do something like that.

But given that we already wrap the C locks in Rust abstractions that implement a
common trait (interface), we can use Rust generics to leverage all locks without
the need for macros.

If you have a particular sequence lock that is shared between Rust and C
code, it would be good to be able to easily to find the Rust uses given
the C uses and vice versa!

I am not claiming that generics won't work, but instead that we still need
to be able to debug the Linux kernel, and that requires us to be able to
quickly and easily find all the places where a given object is used.

Fair point. We need to spend more time on tooling to link the C code with the
Rust wrappers and the usage of wrappers.

Very much agreed!  ;-)

quoted

RCU pointers can be implemented with a similar technique in that read access is
protected by a 'global' RCU reader lock (and evidence of it being locked is
required to get read access), and writers require another lock to be held. The
only piece that I haven't thought through yet is how to ensure that pointers
that were exposed with RCU 'protection' cannot be freed before the grace period
has elapsed. But this is a discussion for another time.

Please note that it is quite important for Rust to use the RCU provided
by the C-language part of the kernel.  Probably also for sequence locks,
but splitting RCU reduces the effectiveness of its batching optimizations.

Agreed. We actually use the C implementation for all synchronisation primitives
(including ref-counting, which isn't technically a synchronisation primitive but
has subtle usage of barriers). What I mean by "implemented in Rust" is just the
abstractions leveraging Rust concepts to catch misuses earlier where possible.

Might I suggest that you instead say "wrappered for Rust"?

I am not the only one to whom "implemented in Rust" means just what
it says, that Rust has its own variant written completely in Rust.
Continuing to use "implemented in Rust" will continue to mislead
Linux-kernel developers into believing that you created a from-scratch
Rust variant of the code at hand, and believe me, that won't go well.

That's good feedback, thank you. I'll police my usage of implement vs wrap.

Just for my education in the other direction, what do I say to indicate
"written completely in Rust" as opposed to wrappered when talking to
Rust people?

							Thanx, Paul

quoted

For at least some of the Linux kernel's RCU use cases, something like
interior mutability may be required.  Whether those use cases show up
in any Rust-language drivers I cannot say.  Other use cases would work
well with RCU readers having read ownership of the non-pointer fields
in each RCU-protected object.

Again, I did add rough descriptions of a few Linux-kernel RCU use cases.

quoted

I'll send out the patches for what I describe above in the next couple of days.

Does any of the above help answer the questions you have about seqlocks in Rust?

Possibly at least some of them.  I suspect that there is still much to
be learned on all sides, including learning about additional questions
that need to be asked.

Fair point. We don't know quite yet if we've asked all the questions.

My main immediate additional question is "what are the bugs and what
can be done to better locate them".  That question of course applies
regardless of the language and tools used for a given piece of code.

quoted

Either way, thank you for your work on this!

Thanks for engaging with us, this is much appreciated.

Cheers,
-Wedson

quoted

							Thanx, Paul

quoted

Thanks,
-Wedson

quoted

So the trick is to stage things so as to allow people time to work on
these sorts of issues.

quoted

In any case, Rust does not necessarily need to help there. What is
important is whether Rust helps writing the majority of the kernel
code. If we need to call into C or use inline assembly for certain
bits -- so be it.

quoted

But to be fair, much again depends on exactly where Rust is to be applied
in the kernel.  If a given Linux-kernel feature is not used where Rust
needs to be applied, then there is no need to solve the corresponding
issues.

Exactly.

Thank you for bearing with me.

I will respond to your other email later,.  but the focus on memory
safety in particular instead of undefined behavior in general does help
me quite a bit.

My next step is to create a "TL;DR: Memory-Model Recommendations" post
that is more specific, with both short-term ("do what is easy") and
long-term suggestions.

							Thanx, Paul

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